Are 3% SH plans exempt from Top Heavy

Plan is 3% nonelective Safe Harbor plan.  Only SH & deferrals.  TH ratio is 80%.

Is this plan exempt from Top Heavy?

"Exempt" is a word I don't like to use on this issue. If the plan meets all the requirements, it is "deemed" not to be top heavy. Probably just semantics, but even such a plan could be subject to top heavy if, for example, eligibility is different for deferrals and Safe Harbor contribution.

P.S. - you may find Revenue Ruling 2004-13 helpful.

https://www.irs.gov/pub/irs-irbs/irb04-07.pdf

does it matter? the 3% SH satisfies your TH minimum and is fully vested. 

It might. Suppose the plan uses comp from date of participation - your 3% safe harbor won't satisfy top heavy if you became eligible for the SH mid-year.

That's one of the good results a "safe harbor contributions only" gets you, though.  Only 3% of half the year's pay.  Or a $0 match because the participant made no deferrals.

(And yes, sure, there could be other circumstances that cause this not to hold, but if it's a single employer's only plan, and as "plain" as you're representing it, you should be good.)