Perfectly fine.

They don't have the modern high ceilings of T5, but they are fine and decorated to the same standard.
Personally, I prefer the Galleries Club at T3 to the one at T5.
And I don't mind which Galleries First I use, T3 or T5.

What cabin will you be in and/or what status do you have?
- Keep in mind at T3 you also have options (depending upon time of day and entry requirments) of using the AA and Cathay lounges.
And T3 has 3rd party lounges as well (paid, priortiy pass, etc).

So, I'd generally say it is better served in terms of lounges than T5. While the BA ones are certainly smaller, they are perfectly fine and (IMO) to T5 standards.