If you don't like any, you'd have to pay more
This is exactly what you should say.
Now, prior to creating the logos, you should have a design briefing meeting with the client, so that the client can give you some direction and you're not just striking out blindly with your three designs.
I like to give homework by asking "What are three (sites, logos, brochures, etc.) which you like? Why do you like them? What are three etc. which you don't like, and why?" Listening to what your client does and doesn't like will give you a lot of information, even if the client can't articulate "I want a responsive flat design in cool colors and understated serif type because we're financial services."
As far as wording:
Designer will provide three choices of logo for Client's review. Client will select a logo. Client has up to three rounds of revisions to refine the logo design. Additional rounds will be billed hourly. Designer's hourly rate is per attached contract.
(obviously with your own words, but that's the idea.)
You do, in fact, want to scare them — not with the possibility that they won't like your idea(s), but the possibility that if they don't give you decent initial direction and good feedback, you are going to charge them for going 'round the mulberry bush. You are giving them clear warning to get their collective act together and not waste your time. You are a professional; your time is not free.
Attitude helps a lot here. If you've already spoken with them and presented a clearly helpful, "we're partners in this" attitude, then this clause shouldn't be an issue.
