Combing a 3 band IR geo tiff with a 3 band RGB tiff to create a 4 band RGB+IR tiff

[ChrisCrowe1]

Here I have a 3 band IR Tiff image.

and I also have a 3 band RGB Tiff for the same area.

How would I combine the 3 IR bands into 1 new band and place the new 1 band into the 3 band RGB image.