I think I know what the answer will be but just to check with the knowledgable beings.

I want to get off my couch and do soemthing for fun and execising so I want to get a bike. Which one should i get? i dont know people who ride so I dont have anybody to go with me when looking at these. I saw them on the local classifieds.

> mongoose nx 7.1 - (i looked at mtbr and it seems so be a good bike, older gooses) $275 or better, i thnk i can get it cheaper
> specialized hardrock xc (dont know the year) $300
> jamis komodo 1.0 (again dont know the year) $300

i live in puerto rico so the bikes are a bit more expensive than what i could get in the USA so dont explode on the amounts.

That Mongoose looks like it hasn't been made in a while, so it's an older bike. Would probably be my last choice.
The HRXC and Komodo are two different animals. The HRXC is the more race-y of the two Hardrock models (Hardrock and Hardrock XC, Specialized entry level hardtails). The Komodo is a slack all-mountain hardtail, with the newer models being burlier than the older models.

So what do you want the bike for? Are you going to be riding it around your neighborhood? Fire roads? Singletrack with a lot of climbing? Go with the HRXC. Or are you looking for more of a playbike to throw around and jump? Don't get me wrong, the HRXC will take some abuse too, but the Komodo is made for that. Similarly, the Komodo can climb just fine, but it's not the billy goat an XC bike will be.

And more importantly, are they both your size?

Yes they are all my size, well i think i know whats my size.

I want to do everything. I dont think I will be doing difficult trails any time soon, but some singletracks, wood trails and around the neighborhood. I guess the jamis and specialized give me more upgrade capability.

So Hardrock?