Hello, I have a problem. I wish to be able to use items from Fallout 3 and Fallout New Vegas in the GECK. When I try to load both of them in the data it just crashes. I can't find a way to import Fallout 3 items into GECK or vice versa. I wish to have access to all the items.

I believe there were some minor changes made to the files for New Vegas, enough that the NV GECK doesn't like the Fallout 3 ones and vis a versa

HOWEVER, as Bethesda forbids the use of assets from one game in another (Even when both games are theirs, and New Vegas was made by Obsidian anyway) then you may struggle to find a guide on these forums as such could result in someone else breaking the rules.

There are guides (For converting 100% custom stuff made by modders), but I've never seen one on the forums, possibly for the reason above.

Is it possible to import custom 3D Models?

You can certainly build your own models and get them into the game, and if you can convert a custom model into a NIF then you can put it into the game. However the process is not straightforward. There are quite few tutorials about that show how to build a simple model and get it working in the game, have a search around.

I have a question, how do I set a player spawn? I am planning on making a new map but I need a place for the player to? spawn, create character and so on.

Basically you have to create a startup quest and use its scripts to set everything up. Set up an x-marker for where you want the player and use player.moveto to move them there. Take a look at quest CG00, which is the start-up for Fallout 3.