Comments on all streets are welcome.

3/6 Limit Holdem
6 players
Converted at weaktight.com

Stacks:
UTG ($159.75)
UTG+1 ($177.38)
Hero ($104.50)
BTN ($163.00)
SB ($121.50)
BB ($117.50)

Pre-flop: (1.5 SB, 6 players) Hero is CO J K
UTG calls, 1 fold, Hero raises, BTN 3-bets, 2 folds, UTG calls, Hero calls
Seems to me that cap here is not an options, as well as folding. So I call.

Flop: 8 J 4 (10.5 SB, 3 players)
UTG checks, Hero bets, BTN raises, UTG calls, Hero calls
Because original raiser likely to bet or raise no matter what he has, I donked to try to force UTG out.

Turn: 7 (8.2 BB, 3 players)
UTG checks, Hero checks, BTN checks
After his flop raise I decided to calldown, but now I am thinking (may be that's because I know that he checked) - may be b/f turn was the play?

River: Q (8.2 BB, 3 players)
UTG checks, Hero checks, BTN bets, UTG folds, Hero calls ???

Thank you in advance!

Yeah, preflop is good. The initial raise is for value, and then calling the 3-bet is sensible. (Capping would not be atrocious versus some 3-bettors, as it could create postflop folding equity, but I think your play is the better default by far.)

Your plan is very good based on your assumptions. However, I generally go for a checkraise here. The idea is that the PFR will almost always auto-bet in a 3-way pot but may not auto-raise (meaning that if you bet and get raised, the chances are pretty good that you're in trouble, and in a sense you'll mostly get "protection" when you're drawing and don't really need it). Nevertheless, I will say that the danger of a flop check-through is not completely insubstantial (people seem to be checking their whiffs more often these days than when I first started playing online). And, also, when you checkraise, you will sometimes run into a 3-bet, and that 3-bet will mean you're probably beat but not so often that I wouldn't still call down against an unknown (because, well, some people play AK unimproved or TT versus overcards like the nuts, especially in position). So there are drawbacks to my usual plan as well. But one advantage is that getting UTG to call twice with 77 when he shouldn't even have called once is profitable for you (and he probably wouldn't have cold-called with that hand). Other worse hands can call one profitably, and even if they can't quite call two profitably, they can make that second call after making the first one. But protecting out-of-position has its hazards, as I tried to explain earlier, and sometimes getting someone to call two unprofitably on the installment plan is better than trying to get them to fold for two (since the flop c-bet figures to be close to meaningless, but if you bet and get raised, the chances that you also are drawing go up).

Given the previous action, I would have played the turn the same way you did. The PFR's flop raise was distrubing enough, and UTG's flop cold-call has me worried too. Plus, usually the PFR will bet again. When he fails to do so, I start thinking he's now going for a cheap showdown with a hand like 99. Another possibility is that he raised for a free card, but the flop wasn't very drawish (and, additionally, to some extent the free-card play has fallen out of fashion), so I'm not thinking about that so much.

Yeah, Villain probably has AQ or something, but I think you need to call after this action at these odds. Maybe he's value-betting 99 now that the scary UTG cold-caller failed to wake up (even though T9 got there twice!). Or maybe he's stabbing with AK (after raising his overs on the flop to isolate your "draw") since no one seems to be showing much interest anymore. Anyway, I wouldn't expect to win, but I would call at these odds (9:1), closing the action, and I don't even think it's close.

Well Preflop looks fine to me

Flop I like, this puts maxumum pressure on UTG.

Turn is where I disagree: Think about what his raising range could be here, if he is at all aggressive his range could be 100% of his preflop range. If that is the case he will be taking a free card alot of the time on the turn. Because UTG called flop bet, I would donk the turn. If he raises he should knock out UTG, and you will just c/c down. If he calls you can value bet the river.