The three combo amps I was looking at are the Peavey 5150, Marshall JCM 800, and the Marshall JCM 900. My styles are Hardrock,Metal,Grunge,Alternative Rock, and Classic Rock. Any suggestions would be greatly appreciated .
You need to play them and decide which one sounds best to you.

the 5150 is amasing for metal

the JCM800 is better for more classic (80's) metal

the JCM900 is sorta in between but more towards the JCM800
My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
MXR M132
MXR M101
MXR M108
Probably the 5150 for you. It depends if you want that Marshall UK tone (JCM900/JCM800) or a little more gain and darker 5150. The JCM900 has more gain too.

For you I think its between the 5150 and the JCM900. All depends on if you like Marshall tone or not.
i find that i really like the 5150 and i play all the genres u listed plus death metal
I say the JCM800, I got one recently and love it. I was going to keep the TSL for the metal stuff but although it doesn't have the same high gain as the TSL, the 800 sounds a lot better doing it and I've decided now to sell the TSL cause I just don't use it. You'll nail AC/DC, Led Zep and the Gunners easy with an 800 too. Ok it may struggle with numetal, and it doesn't have Fender cleans but you can't have everything :P
JCM900. Better classic rock tones, by far, than the 5150. Plus a lot more gain than the 800. Its kind of inbetween.