An employer has only a profit sharing plan that is super top heavy. What is the minimum contribution?

Allocations (forfeitures + contributions) must equal the lesser of 3% of eligible comp. or the highest allocation rate to an HCE.

if there is only one plan, the minimum is 3% (or less, depending on what any of the 'key' (not HCE) employees receives.
It doesn't matter if a plan is top heavy or super top heavy
that only mattered if there was a DB plan, and you wanted a 'buy back' on the 415 limit.
with the new regs I don't believe the term matters any more (beginning in 2000)

One addition to Tom's comment:
The "buyback" is no longer relevant because of the repeal of IRC 415(e). But the plans must recognize this repeal, either thru plan amendment or by valid reference.