Can I only subscribe to 3 programs

you can, but that's not usually wise.  and there are no special discounts for subscribing to more than one program (although the photography plan that includes photoshop and lightroom).

because each plan is about $20/mo and the all programs plan is about $55/mo, it doesn't make much sense to pay $60/mo for a lot less, Adobe Creative Cloud Plans, Pricing, and Membership