It is good that you have decided to do b.sc in agriculrure, afterall it is your choice. But as you say that you are weak in phy and maths and these two are most important subjects. Only 3 months left yes it is possible to cover these two subjects especially. As you are from pcm, so you need to cover these three subjects. 

Try to give equal importance to all three subject. With three months you should rather start strenghtening the strong parts. It would be more beneficial.