I want to replace the system on my mountain bike with SRAM parts. From my quick search on the internet Im finding that its hard to find parts if I want to build a 7-speed bike (3 x 7). Im not looking for the most expensive high end parts. I would like to build an entry level mountain bike. I already have SRAM MRX grip shifts but I would like to figure out the bottom bracket and crankset. Can anyone give me some pointers?

If you are looking for an entry level bike, forget trying to build it yourself, as you can't get the economies of scale bike companies can, and whatever you get will be expensive vs a off the shelf bike. Even high end is cheaper to get off the shelf unless you have very specific requirement.

If you did look to build yourself, 3x7 is almost dead even for low end parts, X5 which is the lowest SRAM full groupset is 9/10 speed. although you will probably find it easier to source Shimano, with 2015 Alivio (M400) looking to be the best of the budget groupsets available.

Here is a link to current groupsets, SRAM has X3 as a partial groupset, but only has shifters and front and rear derailleurs, where as all others are complete. Buyer's guide to mountain bike groupsets - BikeRadar

Thanks for that link its really helpful. I guess your right its pretty pointless being that 3 x 7 is almost dead. Really my reasoning for wanting to build is because my local bike shop told me that the system on my bike is old and with mixed parts which is why Im not getting good shifting. That and my chainrings are worn and I been having a hard time trying to find a riveted crankset that would fit my aged loose ball bearing bottom bracket. If I can just get new chainrings that would be my final investment in this bike. Im starting to accept that I just need a better bike.

Any idea where I might be able to find a crankset for my bike? teeth count is 48-38-28 and it has a square tapered iso interface.

Yes, no question a new bike is the wiser choice. If you need to hang onto the bike a while longer then go ahead and get the crankset, but not if you are planning to sell it soon, as you won't get much extra by doing that. As for where to find a crankset, how about you Google 48-38-28 crankset - lots of choices starting at $29.95. As that combo is not as typical on the more modern BB's you will tend to find that they will almost all be square taper.

I'd like to use this mountain bike to learn how to ride on the trails. There is a trail close by for beginners and I just want to make the bike as safe as possible to ride. Dont want my chain skipping while im climbing. After I get the hang of things then I'll sell it. Which is why I would like to hold on to the bike.

Should I be concerned about my front derailleur being compatible with new chainrings? I read on sheldon brown website that most old front derailleurs have greater compatibility or something to that effect.

What if I were to get this one?

Sunlite Crankset Sunlt CTLS STL 28 38 48X170BK4ARM WCG 21837 | eBay

The front derailleur is falcon brand

I should have mentioned that it's best to get a complete crankset including bottom bracket, or order one the supplier confirms will match. Why go to ebay for off-brand Falcon when Shimano is little more - also easier to confirm compatibility.

There's a good chance you'll need to replace the BB or at least spindle.
Newer cranks "TEND" to use shorter spindles.

You aren't confined to a 28-38-48.
I've swapped in a crank that was 22-32-42 in place of the former. The FDER should be changed, since it's a different radius for the larger rings.
However, the Falcon brand isn't very good anyway, even compared to inexpensive Shimano Tourney level FDER's.

Im sorry I wasnt clear. The bike already has a falcon front derailleur. Thats where my concern with compatibility is coming from. I would like love to put this one on my bike with a new bottom bracket:

Shimano M131 Crankset 170mm 48 38 28 | eBay

Is my front derailleur limiting my options?

My local bike shop told me the same. If Im going to change the chainrings then I also have to change the front derailleur. Looks like I got my work cut out for me. But I think its worth it. I been learning alot

If I were to get the Shimano M131 Crankset, how would I figure out what BB would fit my bike?

Already answered:

I used to have a link (no longer works) where one could plug in the part# and it'd give you links to the tech docs/parts list.

IF you have a cup & cone BB, there's a good chance you could flip the spindle and use the short side.
Else, you basically have to try it and see if the range of the FDER will work. IF not, you go longer or shorter.

According to Shimano's techdoc it's the BB-UN26 (-K)