You just have to try to play the board. Usually it makes sense to use the things you have vs what your opponent has. So if you have 3 extra pawns vs a piece try to use them and get a pawn storm going. If you have an extra piece vs 3 pawns try to use that piece actively to help make an attack. But it all depends on what the position is like after the exchanges. 

What if 3pawn for 1 mimor piece exchanged?

What is a "mimor" piece?

It depends entirely on the position at hand. 

You may want to consider, that these point values are not at all static; a knight that lands a royal fork is worth far more than a mere three points just as a bishop that skewers a queen is worth a lot more than three points. A connected passed pawn is oftentimes worth far more than one point. 

Finally, any piece that delivers mate is practically worth an infinite amount of points. 

what

If you have the pawns, trade everything because 3 pawns + king vs. a piece + king gives you a winning chance in the endgame. If you have the piece, use it actively to win some pawns or pieces or even the king

Highly subjective. Depends on the position.

In most instances, esp. in games still in the middle game, I think most people would prefer to play the minor piece up side.

How far down the board are the pawns is another important point. If they are still on f2, g2 and h2, then they are more or less useless and adv. to the guy who is up a piece.

Also which pawns of your opponent (Say A vs B) have been taken. If A has given up a minor piece for 3 of B's pawns, which pawns has B lost. If it is the pawns which allow open lines for B's Rooks to line up A's king, that doesn't sound great for A.

It is only in endgames that having the pawns helps.

So K and 3 pawns, vs K and minor, is great for the guy with 3 pawns, for he cant lose.

Using statistics, they're about the same but, in the games you're actually playing it depends on the situation, always. That said, as a general rule you can think of how much stronger (or weaker) do the other pieces become after the trade, and if the pawns can hold or will become easy targets. In simpler terms, you should think of whom will be attacking (or defending) after the exchange, and if such attack can be stopped or not, because should the pawns survive and reach an ending then will have higher chances to win the game by themselves.

A pair of examples: Should the pawns be those sheltering the own King, it depends if your remaining pieces can shelter him and have some extra material, not in defensive duties, available for a counterattack. Or should the trade leads into an ending, where there again you must evaluate if threatening with promotion will cost the opponent a piece or more, and here again, this is influenced by how advanced are the pawns, if they need protection or can defend themselves, and so on.

OP was enquiring about the trade of 3 pawns for a minor piece.