player #1 goes all in Player #2 goes all in with more chips player #3 starts talking about the hand, can player #3 be penalized (only 3 player in the hand)
I think you'd find that enforcement of this would vary widely from room to room. While it's true that player 3 is the only player left to act and cannot influence other players' action, it's still likely to be ruled illegal. Any penalty that might be applied is likely to be making the player sit out a number of hands. I would not expect his current hand to be ruled dead. And I'd really be surprised if any penalty were applied here unless the player is a habitual offender in cases like this.
From my experience dealing it depends. Not only on the venue/event, but on what the player is saying. In the WSOP as long as the player doesn't actually say I have X,Y in my hand and there can't be anymore significant action, I.E. their talking can't effect a player who hasn't acted, it is fine. It's frowned upon, it's considered rude by many, likewise it's considered a valid tactic by many too. If unsure ask the event/venue's rules if you're a person who likes to talk during hands. Some don't mind once it doesn't effect others, other places have a zero tolerance policy on it.
You're not going to get a slap on the hand or a penalty(in some places) for talking your thoughts out aloud, once no more action can happen and you don't specify your hand. You will get a penalty if your talking affects other players yet to act or if they're further cards to be dealt.
As for penalties, you might get a warning if the TD isn't strict, you might get a round penalty. A round penalty is the most common in the bigger casinos and events. Local casinos you'll probably just get a hard time from the table regs and the dealer or floor ask you not to continue talking in a hand. You're hand will never be made dead though.
It's bad form unless you are heads up.
If you do say something, you'll probably get a warning ("there are still other players, please don't discuss the hand") and then a penalty if it is noticed (or someone makes a big deal about it), especially if it's clear you are attempting to influence the action.