Whats an easy way...

to suggest a break to my boyfriend of almost three years. How can I bring this up without breaking his heart?

You should be comfortable with him enough by now to sit down and talk to him. Explain how you are feeling and know that either way, it's going to hurt him. He's going to second guess everything he's done and think about everything that could have been. Have valid reasons for why you feel that you need a break.
Sam

There's no easy way. Be honest and true to him. Even though he probably will be heart broken no matter how, he'll have the comfort that you were honest.

There is no easy way. about a year ago I suggested a break to my boyfriend for about 2 weeks and it helped a lot, but telling him took a lot of courage and was painful for me and him.

I told him - (firstly phoned him and told him that I needed to speak to him and that he wasn't going to like what I had to say) and then wrote him a letter about it, so he could understand why, just because I got to emotional speaking to him about why.

And it helped, we recently celebrated our 3 year anniversary and are closer then ever.