Question

Boron cannot form B 3+ ions.Why?

Answer 1

it is easier to remove the single electron from 2p sub shell to for B^+. 
But it is difficult to remove the electrons from stable 2s subshells which is completely filled. so higher ionisation energy is required. moreover due to smaller size of boron atom, the 2s subshell is more compact and there is larger attraction between nucleus and the 2s electron. So again higher ionisation energy is required.. so B3+ does not forms easily.

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Answer 2

Boron does not form B3+ ions, because of its small size it required high energy to remove three electrons from boron atom.

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