no, you're wrong.
If you take an image recorded with 5D3 and crop it to the size of a 1.6x APSC sensor you would only retain a resolution of about 8MP. You can calculate it like this:
Size of FF: 36 x 24 mm
Size of APSC: 22.5 x 15 mm
Both sensors have a 3:2 ratio
Resolution 5D3: 5760 x 3840
5760 Px * (22.5mm/36mm) = 3600 Px horizontally OR simply 5760 Px /1,6 (crop factor) = 3600 Px
3600/3*2= 2400 Px vertically
So the APSC-Crop will have a resolution of 3600 x 2400 Px which is 8.64 MP, which is less than your 12MP 450D.
To fully answer your question:
12MP -> 4272 x 2848 Px
5760/4272 = 1,34
If you were to crop the FF image to 12MP the crop factor would be 1,34.
(I hope i could help you a little with your maths skills
BUT: Still the quality of the 5D3 image is so much better (sharper) that i can assure you you will not loose any detail. I had a 550D before (18MP) and upgraded to the 5D3. Since i do a lot of macro shooting i was really concerned that because of the weaker magnification the 5D3 can deliver i would obtain lesser detail. But the truth is that the images i get from 5D3 are totally crisp and sharp on pixel-level which the 550D wasn't to that extent, so i get about the same level of detail.
MPs are not the only deciding factor if you worry about IQ (or even sharpness). The 5D3 will do so much better in that regard than the 450D, you don't have to worry about losing your 'crop factor'.
My advice: UPGRADE!