GIMP :: Making Collage Printed On A3 Size

I am trying to make a collage which will be printed(=300 DPI) on A3(420 × 297 mm) sized paper. The images I want to take elements from for my collage are all different in size and DPI. When these elements come together they should be similar in size in order to create a new believable scene. 1 element can't be absurdly large in relation to another.

So what I did for my first image (size of 3456 × 2304 pixels, a print-size of 1219,20 × 812,80 mm and DPI 72 × 72) was increase the DPI to 600 in order to obtain a smaller printsize - reducing it to 146,30 × 97,54 mm - cut-out the elements I need and import them into my A3 document. This gave me the size I need for my elements.

However when I try to take elements out of the second image (size of 1500 × 1125 pixels, a print-size of 211,67 × 158,75 mm and DPI 180 × 180) - which I leave unchanged because 211,67 × 158,75 mm is about the size I need for the elements - and place them into the A3 document they are too small. Same story for the third image (size of 2400 × 1600 pixels, a print-size of 304,80 × 203,20 mm and DPI 200 × 200)