Wow, Chryssy that's my car (well almost, I have a 2009) and I would never have thought I could fit that much in there.
I know the KF30 will fit in the center nicely with the base beause it's quite narrow, and the built in lock offs make it really nice and snug. What kind of Graco is it? If it's a SN32/35 the base flairs out quite a bit and is very wide, which may or may not fit next to the KF30, but for sure it will fit baseless (I've done this before). The BLVD might be a problem, but it might not. I can't be sure because I've never had one in my car. I know that Cosco Scenaras are somewhat narrow and they're very affordable, so maybe I would look into getting one of those for your 18 month old just in case the BLVD doesn't fit.
So in short I would do the Britax/Scenera-KF30 with or without the base-Graco baseless. I wouldn't try to put the BLVD in the middle.