In a company with multiple founders, it's definitely common for an investor to have more equity than a single founder. If it weren't, then you'd be limiting your investors to about 15% of the company.
The main thing you want to worry about is giving control to your investor. If he gets 30%, then he still hasn't hit 51% and can't control the company by him/herself. But, it does mean that the investor could pair up with 2 of the founders to control the company, since they'd add up to 58%. So, how tight are the five of you?
Recall, though, that the company is managed by the board of directors. So, the control isn't direct -- the investor and the two founders would elect the board, and then control the company through that elected board. So, the goal should be to keep that scenario from happening.
So, what can you do? With any investor, you should have a discussion and agreement about board composition. For example, the investor might get to name one board member, and the founders get to name the other two. And then you'd write up things that have to be unanimous -- going into a completely different line of business, for example. That keeps control with the founders, but gives the investor the ability to be part of the conversation and to veto major changes.