So, noob question. But is it possible to wear more than one 3D item at once, and if so, how?

as long as they are not made for the same bodypart yes it is possible.
if not the same bodypart its no
unless you find a glitch or smthing

Short story, no its not.

long story
Some items have been combined as of popular demand like in the swagtastic gear.

edit: alpha beat me to it.

Oh okay. So as long as the items don't overlap, it should be fine?

I should more say as long as they aren't on the same body part. So hat and blindfold don't work, but would kick n kicks and a hat work?

yes it will work.
you can wear 1 item per bodypart and 1 item per joint.
example:
barbed wire + little latching elf + both kickin kicks + swagstatic gear + both elbow pads + mecha arms...ect

you already got it anyway

It's possible to stack few items on the same part if you put all items in a set and then activate it. not sure if it's against the rules to do that though..

nah, personally I'd encourage people to find these kinda glitches. It can make some neat things happen.

Shuttershades colour could change with different items being switched with it.

sometimes you'll get one of the two items showing, alternating between themselves.
sometimes you'll get one item with a different colour (or a mix of both textures) that doesn't change otherwise (shuttershades being an example)

and the rare outcome, deformed/mix of two items and textures.
I recall seeing one combo where the tori ended up with a deformed version of cat ears, unsure what the other thing was though.

Yes
Like what AlphO said
1 Item per bodypart...not per joint.

I thought that set glitch only worked with hairs. Gotta try some stuff out now...

It only works with hair items