Is it by rules (texas holdem) to show hand before calling fold or "all in" in situation when two players are in showdown ? In more details ... there are three players on the table, on river first one is calling "all in", second players folds ... can last player first show his hand and then call or fold ? Since there are no more moves showing hand can't disturb anything so it sounds to me that it can be legal ?
You need to be careful with this. In some casinos this is perfectly allowed while in others, like most here in Vancouver, your hand is automatically dead the second you show it to another player, regardless of how many people are still in the hand. This is because it's a form of angling. You are trying to gain information from your opponents response to seeing your hand.
In other casinos, like in Calgary, it's actually a fairly common, and allowed, practice. Just make sure to ask the dealer before you do so.
There is also the Jamie Gold rule which states that you aren't allowed to discuss, or show, the contents of your hand with our opponents at the table. This rule is strictly enforced during World Series of Poker events. This rule stems from Jamie Golds actions during the 2006 World Series of Poker.