It also really depends on how comfortable you feel with the champion you're playing. For example, this season I havea 10/12 won games with Maokai. Even though it isn't a big number, the win rate is very high to the point that I feel it's good enough to be a trusted champion. Last season, I had my most games on Viktor (62 games I believe) with a 39/23 win / loss ratio. I believe that's a pretty awesome winrate and there are enough games to feel good about it.
3 games isn't a lot, but you can know if you feel confident / comfortable playing it, thus the 100% win rate being ok. I can't tell you an exact number, but starting at 10+ games, you can make assumptions.