Is this my leak?

[Cerril]

I've been working on leak diagnosis and I'm pretty certain it's postflop play. Right now I'm trying to decide whether I'm calling down too much or not being aggro enough (second and third bets) in similar situations.

Here's one that made me pause, as it comes up a lot and I tend to be behind often enough...

top pair middle kicker (something like JQK with a 9 or T kicker), nonthreatening board. On the turn I get checkraised (opponent has 1.5-3 AF) and call down. Is this an autofold against someone for whom I don't have notes that they make this sort of bluff?

I catch enough bluffs here(usually with better hands) to make this a legitimate question, but I feel like I'm up against two pair or at least a better kicker often enough to send me here.

Also, what's a three-betting hand for most of you in this situation, readless and with numerical reads (provided you haven't seen them in this situation yet)