One AS Level with 3 full A Levels?

What are you planning on studying at university?

My "dream" is doing a double major in Math and Chemistry. I'll prefer Chemistry tho

Then I suggest you take double maths and see how it goes, you can take it as a fourth full A-level if you find it managable. It's not uncommon for people to take it as a fourth

My daughter did the same a'levels to get in for a physics degree for 2020 start. The first term of year 12 was full on with the FM. She was not sure whether or not to drop it. I advised her to keep on at it to the end to get her AS and see how she felt after that. She got a B in the AS for the Further Maths and so she decided to keep on for the full A'level. She glad shes done it now (she was still predicted a B before covid came about) so yes its worth going for and sticking to the AS level, then make a decision August 2021 if you want to carry it on.

If its for a Physics/Economics/Engineering degree the FM is worth it

I hate to say this, but I have only 2 months for prep . I'm taking them as a PRIVATE CANDIDATE. I actually belong to an alternate curriculum (national). Now the usual Math seems really easy. I'm confident I'll get A* in it.

I really have to struggle in Further Statistics part. I was wondering, should I take my usual 3 A levels, and take the FM only up to AS level? I might then opt for an A there. Or should I just take the 3?

Canada applications are in January 2021, and I'll take them in November this year

Take the FM up to AS.
My daughter didnt get on with network flow. I think most people have a weak topic, but that's ok, its worth a shot just to get the AS And if your uni course does have some further math elements like vectors, matrices, calculus etc you have a bit of a head start in undergrad