My goal is to make a large rectangular cake. I will use 6 cakes that are about 12x16", 2 side by side and 3 high. I want what looks like one huge cake. I was going to use regular frosting colored with icing gel color to hold them all together. I have never tried this with so many cakes and would be devastated if for some reason they fell apart. Will I be all set to stcak them 3 high using just the frosting as a glue?

I've done it with four and it worked fine. It's really important to have a solid base so they don't split down the middle when you transport it.

By 'regular frosting' if you are referring to buttercream then yes, using buttercream is fine. I would however suggest doweling them at every layer. It will not only make serving easier, you will not have to worry about the weight of the cakes on each other.

I would think your biggest issue will be moving it. If you use a crusting buttercream and your board underneath isn't strong enough, it will crack and craze. You are going to need a least a 3/4" piece of plywood under that beast (this of course only adds to the total weight of it) and I'd be sure it will fit thru doorways before you assemble it.

By "regular frosting" do you mean cans of frosting from the supermarket? Will you be using the frosting between the layers too, or a different filling?

In terms of keeping them together, you can use straws or dowels going straight through the three layers if you're concerned about them shifting -like if you have a long distance to drive with it.

But otherwise, gravity and frosting should be enough to keep it from spontaneously falling apart- as long as they're made from a sturdy cake recipe (try searching wasc if you're not confident in your from-scratch baking skills) box mixes made as directed on the box tend to be pretty fluffy and crumbly in my experience and not good for stacking.

Back to your frosting question... frost it smoothly with an offset spatula and refrigerate it to firm up the frosting and you should be fine. There are tons of you tube tutorials on smooth frosting techniques.

Yes I do mean frosting from a can, however I was thinking that I would add some conf. sugar to it for the sticking of the layers. I will be putting the cake together where the party will be bc Im to nervous to transport it. I have never used anything to support it before..Do I need to even though I wont be moving it?

No- you shouldn't really need to support it if it's not being transported. You shouldn't need to add anything to make it stickier.

Generally, supports should be used for every 4" of cake height to prevent the weight of above cake(s) from smashing the lower ones.

The only thing I can add is to be sure your cakes are level and you'll have no problem at all. If they're humped in the middle you'll have cracking of the upper layers.

I think this cake is four layers high, not four tiers high, correct? If each layer is 1" tall for a total of 4" tall, then you won't need support but you will need a very sturdy base board.