The ORIGINAL CFO I was introduced in 1993 and at the time was slightly larger than the original CFO II that had the following specs:

Diameter 2.56", weight 2 oz. and a line capacity of either a DT3F or WF4 and MAYBE 10 - 15 yards of backing. It was the first reel I ever owned and I used it with WF4F.

The Abel CFO I with a stated diameter of 2.75 is most likely the same size as the original 1993 CFO I. I also own that reel and I use it with a DT3F and MAYBE 20 yards of 20 lb backing.

Bottom line, a DT3 should fit on an original CFO I.

I have a BFR made CFO I, and it currently holds a WF3 on it and as stated above a few yards of backing. It is very easy to overfill, so be careful, but a 3wt line will fit.