Were the grievances of the 3rd Estate (prior to the French Revolution) valid? Why or why not?
Before delving deeper into the question posed, I would like to discuss a bit about the nature of the Three Estates themselves, because when we talk about them, we tend to picture something like the Nobility or the Clergy or the Third Estate as a sort of uniform collective. However, I have to ask: is this really accurate? Take the Nobility: when we talk about the Nobility, what are we really talking about? Are we talking about the high aristocracy that would gather in Versailles? The Nobility of the Robe which served as the magistrates of the Old Regime? The captains and commanders in the French armies? Impoverished feudal lords holding onto past glories? There's a wider range of experiences and perspectives than many tend to expect. This same criticism applies to the Clergy (are we talking about a Bishop or a Parish Priest) and certainly to the Third Estate, which quite explicitly covers anyone who is not either part of the Nobility or the Priesthood, and this goes beyond mere majority, and is instead the vast preponderance of the French population. Looking at the Estates System as a kind of homogeneous whole risks erasing the very wide-ranging fragmentation buried underneath.
This being said, regardless of the vast net of experiences and backgrounds subsumed together within the Third Estate, there was far reaching resentment concerning the Old Regime's social and fiscal structures. Even if the specific complaints a Peasant might have made would have been made from a different perspective than the complaints of a Parisian weaver, let alone those of a trained lawyer, during the early phase of the French Revolution, support for the Revolution was extensive.
Finally, we can take a moment to discuss the Estates General and the frustrations experienced by the deputies who represented the Third Estate. The subject of vote by order vs. vote by head has already been discussed by Pohnpei397 and LarryGates, but in addition to the points already made, I will add two critical points that need to be accounted for. When the Estates General met in 1789, the Third Estate received twice as many Deputies as each of the first two Estates (the numbers were 300 for the First, 300 for the Second, and 600 to the Third). In addition, it should be noted: there were more liberal, forward thinking members among the other two Estates, radicalized by the Enlightenment and just as eager to see reform as the members of the Third Estate themselves were. This was part of what made the question of vote by order vs. vote by head such a high stakes encounter; were they to vote together in a single body, the reformists from the other Estates could align with the deputies of the Third Estate, allowing their agenda to pass. Should it proceed along traditional lines, the Third Estate could expect to be outvoted two-to-one. As the situation gridlocked (neither of the Estates, out of their own political interest, could afford to back down on this question) the situation radicalized and eventually dissolved, with the Third Estate declaring itself the National Assembly, representing the Nation of France.
With all this being said, the question emerges, are these grievances valid? I think they were. For one thing, consider that France in 1789 was marked by systemic and far reaching inequality. Furthermore, consider that the Estates General met in the wake of a complete fiscal collapse, and you can argue on purely pragmatic grounds that the Old Regime's social and economic structures had failed to meet the challenges of the Early Modern State. Finally, consider the experience of the deputies themselves in the Estates General, being pushed to adhere to the rules of 1614, even when those rules ran against their own political interests, and even when they represented the vast majority of the population of France, it's difficult to argue that their complaints were anything but valid.
The grievances of the Third Estate were definitely valid. This is because the Third Estate did not have the political power it deserved and it carried more of the economic burden than it should have.
One of the major grievances was that the Third Estate only had one vote in the Estates General, just like the two other estates did. This was true even though the Third Estate made up by far most of the country. This meant that the Third Estate did not have nearly as much political power as it should have given that it was bigger in numbers than the other estates.
Another major grievance was about taxes. The other two estates essentially did not get taxed, or at least not nearly as heavily as the Third Estate did. The nobility and the Church had all this wealth and land and yet the Third Estate was paying most of the taxes. This was clearly not fair.
So the grievances of the Third Estate were clearly valid because that estate was definitely being mistreated under the old regime.
The grievances of the Third Estate were quite valid, as they were the ONLY estate which were taxed under the Ancien Regime. This situation was exacerbated by the crushing debt which the French Crown had incurred as a result of the wars of Louis XIV and also from involvement in the American Revolution. Furthermore, the Crown had sold titles of Nobility, the so-called "nobility of the robe," who not only paid no taxes; the titles were inheritable, so their heirs also paid no taxes. The end result was that the tax base itself was diminished at a time when the tax burden was increasing. Add to this, the 3rd estate constituted a substantial portion of the populace, including the economically important Bourgeoisie; yet it had precious little political power. They were almost always outvoted in the Estates General by the Clergy and the Nobility, which held most of the land in France and all the political power, yet paid no taxes.