When the small blind decides to come along and flat call your opponent’s open buttons raises, it should really widen your 3-bet range as a squeeze, unless your opponent on the button is extremely stubborn, or decently tricky post flop. It’s best to use caution against these kinds of opponents. In general, most of your opponents are going to know that you’ll be squeezing a bit wider, but that shouldn’t prevent you from doing so if you 3-bet a smart range of hands, and keep your 3-bet sizing good.
In general anywhere from a pot-sized raise to about 4.5x your opponent’s opening raise is a good size to enable a high percent of fold equity. You want a high percentage of fold equity to make up for times when your opponents really have a hand, or you get called and have to shut down because of a poor flop texture.
You should already know what most of your 3-bets for value will be, and some of the in-between hands like AJo, KQo, ATs, etc, will highly depend on how you think both your opponents play. You will be in a sandwiched position post flop. It will make your opponent on the button play slightly more honest, but you won’t know until after flop betting has happened if you’ll have position, or you will be out of position. For that reason in some spots against weak opponents, overcalling with suited connectors can play well if you think the small blind will have a decent hand more often than a speculative hand, or flatting with AJo can have value against weak opponents with poor post flop skills.
Some of the hands you won’t be flatting almost ever, but have great squeeze EV (shown in picture to the right), are hands like: A2o–A9o, A2s– A8s, K3s–K9s, K8o–KTo, Q6s–QTs, Q9o–QTo, J8o–J9o, J7s–J9s, T8s+, T9o. Hands like small pairs can become a flat or a squeeze. If one or both opponents are deep, flatting can usually be more ideal, or if both or one of the opponents is really weak post flop, you don’t want to push them out of the pot by 3-betting. If facing two decent regulars, or at least one reasonably tough opponent, squeezing or folding is usually a higher EV line.
You always want to keep in mind that if you build history with any of your opponents or you are squeezing a lot, that your small blind can and should look to trap occasionally with his stronger hands, knowing you will squeeze wide. This shouldn’t prevent you from squeezing, but it should reduce some of your range so that your opponent isn’t looking to exploit you.