I am having trouble picking between chem 1aa3, bio 1m03 and physics 1b03. Can anyone give advice based on the level of interest and level of difficulty in each course? Any of the three will benefit me as I may write the mcat.

Chem 1AA3 and Physics 1B03 will benefit you for the MCAT. HOwever, the MCAT covers many more topics than discussed in physics 1B03. In terms of which course to take, you should choose the one that will be a prerequisite to the upper year courses you wish to take. Chem 1AA3 is a prerequisite for many upper year chemistry, biology, and biochemistry courses. So think ahead to the courses you wish to take later and where you want to specialize before you choose.

pick what ever ur intersted in no point asking for advice
if u like chem take chem, if u like bio take bio, if u like physics take physics
although i might add that bio 1m03 is really bad and boring

Bio 1M03 is possibly one of the most boring courses I've ever taken.
I really enjoyed Physics 1B03, and it was pretty easy.
I'm in chemistry, so I think my opinions of Chem 1AA3 go without saying.