Wow, 3 nights on Samui only leaves 2 nights on Krabi, PP and Lanta. Can't do it baby.
I personally would drop Lanta which visually is the least spectacular, but if you really want to see Lanta you will have to drop one night from Samui.
Impossible to give accomm recs on account of not knowing your budget - what sort of beach you want (quiet, party etc)
Here are some recs on budget accomm on nice beaches not too quiet - Phi Phi, Phi Phi Hill on Long Beach - Lanta, Lanta Palm Beach on Long Beach - Railay, Rapalla Rock Wood on East Railay - Samui, New Hut on northern Lamai.
If you only have a total of 5 days, but want to relax, I wold just visit either Samui, or Phi Phi, not both. If you choose Phi Phi, then Lanta or Railey are visitable, perhaps 1 of the 2, for a couple of nights.
Middle of the range, I can recommend SPP Bungalows on PhiPhi, Lanta Riviera resort on Lanta & Anyavee Railey resort on Railey (east).