Why this position is draw ?. I believed black is winning (3 queens against 1 queen). Black move

It is a draw due to material insufficiency: generally, majority of "3 draughtsmen vs. 3 draughtsmen" endgames are drawn on 10x10 board. The only exceptions are positions where a player can win by establishing opposition or by imprisonment (stalemate).
The problem of 10x10 board under International rules is that 4 kings are needed for winning against 1 king: there are only few exceptions where 3 kings can ensure a win. Moreover, in the king endgames the material conversion into winning is very complicated if a player can't crown his or her draughtsmen into kings.
For this reason, there are more competitive rules, which are available in some national variants. While I'm not native player, I like Argentinian and Thai rules where 2 kings can always win against 1 king because the king must observe restricted capturing by making a mandatory jump on a square next to captured piece.