At the lower limits, pretty much all the poker rooms have soft games. Personally, I prefer to play in the nicer casinos where the drinks are better and you get more wealthy customers deciding to "give poker a try." In Vegas, that means playing 4/8, since that is the smallest game at the nicer casinos. I prefer Wynn, but Venetian and Bellagio are also good choices.
If you want to play 3/6, I'd suggest playing where that is the smallest game in the room. I know that Caesars spreads 3/6 as their smallest game. I think that Mandalay Bay has 2/4 and 3/6, but its been a while since I've been there. MGM Grand also has 2/4 and 3/6 and I like playing there.
Honestly, the games from 2/4 to 4/8 are all pretty good, so it is really about picking the most convenient spot to where you are staying and finding a place with the vibe that you like.
In terms of time of day, pretty much all poker rooms are easier to beat late at night and on weekends, especially if you don't join the table in downing lots of booze. My wife reports that old guys don't bet without the nuts on the river against her or other women, but that young guys like to try to bully her.
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