I'm 15, so pretty much the only reason I play my Diamond still is for sentimental reasons

. My goal when I was a little kid playing Gold/Crystal was to get a set of 6 lvl 100's, all awesome, and all the best they could possibly be. Now, I have 3/6 of them, and want to complete that set. However, I don't know what other ones to get. Here is what I have so far:
Blaziken
Flygon
Empoleon
I've been leaning toward getting a Venusaur and a Pidgeotto, but have no idea otherwise what to get. Here's the catch- only 3-stage evolves, and I don't want to repeat any types (ex: Empoleon is part steel, so I won't take a Metagross).
Can someone please help me out on this? I'm willing to give out legendaries for lvl 5's of the Pokemon...