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.