If I understand correctly, you are doing Cluster sampling actually, stratified you take a SRS from each strata where as you are only doing 2/3.
You had 3 groups and you randomly selected 2 groups to survey (even if you didn't, you should say you did). The problem with cluster in this case, is you only have 3 groups (clusters) and if their are regional differences within the 3, you won't get accurate results. Assuming you pick a proper sample size from each cluster and say the clusters were picked randomly, the only criticsm I can think of is: "Do you have evidence to suggest that Province X would have similar results to province Y and Province Z. Otherwise you are putting to much weight on X and Y's opinions but none on Z and they would almost surely have different opinions since they are from a different region.")
In other words, would all provinces (clusters) have similar results OVERALL. Can you say ALTA, ONT and NFLD's (just made these up, I don't know which ones you are using) professional associations will have similar opinions. That is the only setback I can see. If you have reason to believe yes and can explain your way out of it, I say what you are doing is fine.
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