Concatenation videos with different resolution. Part 3. Blur

• Concatenation videos with different resolution. Part 1. Pad
• Concatenation videos with different resolution. Part 2. Crop
• Concatenation videos with different resolution. Part 3. Blur

One more way – use original size of videos ( scale only if video size large, than our canvas ) but add blurred background.

ffmpeg -y -loglevel warning
-i 5sec_640x480.mp4
-i 5sec_1920x1080.mp4
-i 5sec_720x1280.mp4
-filter_complex " 
[0:v] split=2 [video0-1][video0-2];
[1:v] split=2 [video1-1][video1-2];
[2:v] split=2 [video2-1][video2-2];
[video0-1] scale=w=1280:h=720,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1, setsar=1  [bg0];
[video1-1] scale=w=1280:h=720,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1, setsar=1  [bg1];
[video2-1] scale=w=1280:h=720,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1, setsar=1  [bg2];
[video0-2] scale=w=min( iw\, min(iw*720/ih\,1280)):h=min( ih\, min(720\,ih*1280/iw)), setsar=1 [video0-2-scaled];
[video1-2] scale=w=min( iw\, min(iw*720/ih\,1280)):h=min( ih\, min(720\,ih*1280/iw)), setsar=1 [video1-2-scaled];
[video2-2] scale=w=min( iw\, min(iw*720/ih\,1280)):h=min( ih\, min(720\,ih*1280/iw)), setsar=1 [video2-2-scaled];
[bg0][video0-2-scaled] overlay=x=(W-w)/2:y=(H-h)/2 [video0];
[bg1][video1-2-scaled] overlay=x=(W-w)/2:y=(H-h)/2 [video1];
[bg2][video2-2-scaled] overlay=x=(W-w)/2:y=(H-h)/2 [video2];
[video0][video1][video2] concat=n=3:v=1[v]" 
-map "[v]" -an -c:v h264 -crf 18 -preset veryfast -f mp4 output.mp4

or version with concatenation audio streams too

ffmpeg -y -loglevel warning
-i 5sec_640x480.mp4
-i 5sec_1920x1080.mp4
-i 5sec_720x1280.mp4
-filter_complex " 
[0:v] split=2 [video0-1][video0-2];
[1:v] split=2 [video1-1][video1-2];
[2:v] split=2 [video2-1][video2-2];
[video0-1] scale=w=1280:h=720,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1, setsar=1  [bg0];
[video1-1] scale=w=1280:h=720,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1, setsar=1  [bg1];
[video2-1] scale=w=1280:h=720,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1, setsar=1  [bg2];
[video0-2] scale=w=min( iw\, min(iw*720/ih\,1280)):h=min( ih\, min(720\,ih*1280/iw)), setsar=1 [video0-2-scaled];
[video1-2] scale=w=min( iw\, min(iw*720/ih\,1280)):h=min( ih\, min(720\,ih*1280/iw)), setsar=1 [video1-2-scaled];
[video2-2] scale=w=min( iw\, min(iw*720/ih\,1280)):h=min( ih\, min(720\,ih*1280/iw)), setsar=1 [video2-2-scaled];
[bg0][video0-2-scaled] overlay=x=(W-w)/2:y=(H-h)/2 [video0];
[bg1][video1-2-scaled] overlay=x=(W-w)/2:y=(H-h)/2 [video1];
[bg2][video2-2-scaled] overlay=x=(W-w)/2:y=(H-h)/2 [video2];
[0:a] anull [audio0];
[1:a] anull [audio1];
[2:a] anull [audio2];
[video0][audio0][video1][audio1][video2][audio2] concat=n=3:v=1:a=1 [v][a]
" -map "[v]" -map "[a]" -c:a aac -c:v h264 -crf 18 -preset veryfast -f mp4 output.mp4

Result: