There isnt much support for the axel if its held in at one point like that
If it's the center hung support for steering, it just needs to be done right. A small plate could by mounted with a bearing between the chassis and the plate for the vertical load and perhaps one or two for keeping the vertical steering "pin" in check. Then mount the 3mm axle on the plate.
It wouldnt be very economical to buy just 4 collars and i dont know where i would buy them in the first place
You can use electricians wire blocks (the proper English term escapes me) - those with 12 screw connectors in a block (usually white/semi-translucent, but comes in black too). Unscrew the two screws of each part and push the metal thingy out of the nylon (reinstall the screws before they go AWOL).
One of these each side of the wheel and a washer of a suitable size between connector and wheel should do.
These blocks are cheap (you may have some allready), comes in different gauges and you could even split them in half with a hacksaw, some patience and some bandages
They're good for joining axles as well and for connecting a motor to an axle.