I have done the Ref 3 upgrade circuit. Namely, the Ref 5, Ref 5 SE and now the Ref 6. IMO, there was an incremental and noticeable improvement from the Ref 3 to the Ref 5SE and then again to the Ref 6.
I recall seeing a few Ref 3s listed on A'gon for the high $3ks to maybe $4K range. The list prices for the Ref 5SE are all over the place, but assume something in the high $5Ks or low $6Ks. So, moving up to the Ref 5SE could cost some change.
That said, I recall that my Ref 3 was a real sweet sounding pre. If you are happy with your Ref 3, especially after the retube, why not stay put and just enjoy it?