Will this fit?
 

I was thinking of going deaf at an early age so i was thinking of getting 3 12in Kicker L5's with Wal Mart brand enclosures so i was wondering if they would all fit in my 06 Mazda 3 sedan?

no better way to find out than to try. j/k, i would get the dimensions off the enclosures and start measuring your trunk space. most people with 3 subs go with a custom enclosure and if you're handy the enclosure will probably be higher quality and sound much nicer.

for the enclosure should each sub be seperated or all together, like when they are all in the box, should there be something in between each sub?

With three subs you are way better off building your own box. My old car i used 3 subs and had a 12 hitting into two 10s, all i have to say about that is rattle control bacame obsurdly expensive

i would leave the box wide open instead of making separations, so that the cones can move and the air can move. sectioning off the sub box i think would be restrictive. research the subs you want and calculate the amt of internal volume (air space) it needs so you know how big your sub box should ideally to be, and make sure you have that kind of trunk space.

the subs i want generally need a box thats is about the size of 2 1/2 boxs, but the same sub i want was seen at circut city in a Walmart brand box. As for sectioning off the sections for each sub, one of my friends had 3 subs with a universal box for 12in subs and they werent sectioned off and they sounded like one 10in

the only reason i dont suggest separate chambers in the sub box is because of space in the trunk. a chambered box can produce better bass, but i think it would be bigger than a single box, and as long as you design it right it should thump the way you want it to. i could be wrong, its been a while since i did any car audio stuff, im just more concerned about practicality than loudness.

the sube im getting are going to be 12in kicker L5's

You should always have a seperate chamber for each sub. The sound is clearer, louder, and it is actually safer for the subs themselves. There is a lot of acoustic mathematics that I could write in here to explain, but I figure it is easier just to tell you. The extra volume of the box will be well worth it. In case you are interested, I have 2 12" Daimond Audio subs in a custom slot ported box for sale.

Cool gotta pic?