@ 25BB the population 3x range is generally the top of their range. I actually insta mark as fish when I see an opponent 3x and also assume he will have transparent bet sizings in the rest of his game too (these guys tend to turn their hands face up in their bet sizing choices). 

With A2o, you're often going to be dominated by the strong Ax in his range. 

If I haven't seen him open 3x before or if he has folded or limped any buttons (indicating that his 3x range is likely to be even further contracted and more dominating vs our holding) I'm simply folding A2o here. 

As he widens his 3x range, you can consider flatting.