You have to adjust your play a lot for heads up and 3 handed. Agression and reading your opponent come into play a lot more, also try and mix your play up as much as possible. A couple things I have found is that at the lower levels be prepared for a lot of hitting and running which can be frustrating but overall not brutal to your win rate (most of these donks will end up loosing to you), there are also plenty of fish at these levels. Overall if you adjust your game you should be able to do quite well and sustain a fairly high win rate.