Mathematics is the compulsory subject to get into DU for B .com(hons) .Few of the colleges has made mathematics compulsory for B .com(prog) too. It's not compulsory thatmathematics should be the part of best of four percentage what matters is that you should have studied it in 10+2.

Hey!

For Bcom honours, Economic honours, Statistics honours and maths honours having maths in 10+2 is compulsory other than that best of four is any three main subjects in which you have scored the highest amongst all five to six subjects that you have, English is compulsory to include in your best of four regardless of the marks scored be it highest or lowest. 

All this information is as per University of Delhi, Eligibilty criteria