(2)3- methyl-1- isopropyl-1- cyclohexene

(3)5- methyl -4- n-butyl-1, 4- hexadiene

When you choose the main chain, you must pay attention to keeping the position of double bonds as small as possible, and then consider substituents second.

For example, the main chain of (3) must choose a chain containing two double bonds, even if it is not the longest.