Generally songs with lots of talkie parts are going to be easier. As the other solution has said, play on medium instead of easy. In my personal opinion, there is almost NO difference between medium and easy vocals.
Short songs are also your friend so you don't have to spend so much time on a song. Plain and simple almost every song is easy to 3 star on medium, and it's almost a given you'll be able to do it. Use overdrive when you have a 4x multiplier, or if you feel like you're going to fail. There's also the no-fail option you can put on which makes you not have to use your overdrive to save you.
Also don't forget, tapping parts will net you some points along the way. I'd stay away from all tier 6 songs if you want to save yourself any trouble at all. But when it comes to vocals, the tiers are a matter of how well you can sing the song.
Example: I can FC (Full Combo: Hitting every phrase in the song without losing your multiplier) Roundabout which is a tier 5 song, but I have a difficult time with Low Rider which is a tier 1 song.
Vocals are tricky sometimes, but once you get the hang of them you'll have it down pat.
Here's a couple of higher difficulty songs that are somewhat easy:
Living in America
Lasso (Very short, and easy to at least 3 star on medium)
Cold as Ice
Caught in a Mosh
25 or 6 to 4
Also, I'd like to mention familiarity with the songs will help a lot. Free Bird is a very familiar song to many and it is very easy to 3 star on medium, but a song like Viva la Resistance, which I didn't know myself to begin with, can be tricky to some.
Don't get me started on Good Vibrations though...
I want to say that you should filter your settings to Rock Band 3 songs only so you don't play any DLC songs you may own.
*You will only get this achievement by 5*ing 25 songs on the Rock Band 3 DISC on easy ( or 3* on medium) *
Good luck on you endeavor to get this achievement. If you're having any troubles, just comment or PM me and I'll try to give you some more pointers.