This is, of course, a matter of opinion. My own view is that these three territories should be granted provincial status. This would lead to some degree of injustice due to the small population sizes of those territories, but the current system is, arguably, unjust as well.
The main reason to say that these territories should not become provinces is that they simply do not have enough people in them. The three territories have populations that are the size of small towns in some provinces. For example, the Northwest Territories have a population of just over 41,000. The other two territories have populations that are even smaller. By contrast, the province of Ontario alone has 30 cities with populations of at least 50,000. It seems wrong to have provinces with such tiny populations. This would be particularly true if the new provinces were to have more representation in Parliament than they do now. The new provinces would be wildly overrepresented in Parliament if that were the case. This would lead to them having more power in the federal government than their populations warrant.
However, I would still argue that these territories should become provinces. I have three reasons for saying this. First, there is already one province that is very much smaller than the rest. This is Prince Edward Island, with a population of only 140,000. There are 32 cities in Canada with populations bigger than that of PEI. If PEI can be a province with such a small population, why should the three territories not get the same status? Second, it should not be necessary to increase the number of MPs that these three territories now have. If the three new provinces were to keep having just one MP each, as they do now, they would not be badly overrepresented. They would still have as many MPs as they have senators, so the constitution would not be violated. Third, and most importantly, giving these territories the status of provinces would be symbolically important. Provinces in Canada are, technically speaking, sovereign political units while territories are run by the federal government. This, in a sense, labels them as lesser political units that are almost wards of the state. It seems unjust to label the territories in this way, particularly when so many of their residents are aboriginal peoples. Given the history of oppression of aboriginal people in Canada, it seems wrong to continue to label them in this way.
For these reasons, I would argue that these three territories should become provinces.