[BluffTHIS!]

With only 3 players seeing the flop, I am betting the flop almost every time. But as played, like the other posters said, you need to know whether you are up against a calling station, and you need to have a sufficient stack size to followup if you are called, which you didn't. The whole thing in big bet poker is not just the size of the current bet/raise you present someone with, but the size of the future one you can put to them as well, especially in the case of a semi-bluff move. And you might as well just save that last $20 on the river as you played it, even if it means check/folding. As Mike Caro has said, money you don't lose is the same as money you win. Oftentimes you will be against players you don't know well, and when such a previously unknown player demonstrates on any street that they are a calling station, then it is just chip spewing to keep going at them.

My comments above about saving that last $20 in a big pot are assuming you are not against a known player so loose that they would be in there with bottom set or top 2 only on that board.