(A similar query appeared here www.videoforums.co.uk/general-software-problems/42904-how-do-i-join-two-videos-more-together.html )
I have not tested this this with your res of clip, but here is an example which I have just written and tested on my pc.
I used 3 clips (1280 by 720), named a.mp4, b.mp4 and c.mp4. I put them into the same folder as FFMPEG; then used the following command to create a single 3840 by 720 clip which contained my original clips, side by side.
ffmpeg -i a.mp4 -i b.mp4 -i c.mp4 -filter_complex " scale=1280:720 ,pad=3840:720 [a];  scale=1280:720 [b];  scale=1280:720 [c]; [a][b] overlay=1280:0 [d];[d][c] overlay=2560:0" -y output.mp4
As you may see, the first section is padded out to be 3*1280 = 3840.
The 2nd clip gets overlayed at position 1280,0. The 3rd clip gets overlayed at position 2560,0.