Dont know if this will help, but..
Waht size do you wan the final pics to be?? Say 5x7
Open all the pics in photoshop and resize to 5x7.((or whatever size)add boarders ect now)
Then with them all still open,
Go file ..new ..
then it will ask you for what you want in size . Prest drop box, Select A3.. OK
This will give you an A3 page, you can maake this whatever colour.
Then select the MOVE tool(V) ,
Click on each Pic and drg onto the A3 page, these will be added as a new layer for each pic. They can be moved into different positions via the layers tool bar.
To ensure they are straight there is a SNAP to tool under VEIW at the top.
Once you are happy you then can just flatten the image to one layer .
Print as usual cause now its one big A3 JPG/TIFf .
I am No expert just worked it out playing around. There is probaly a thousand different ways to do it.
Hope it helps
MATT
Disclaimer: This may or may not work