At BCN? I wouldn't risk it. I assume you don't have European passports, and though the passport queue isn't usually long, it's quite a slog from gate D8 around to the landside, then down to the bus collection area (which isn't well signed, you will need a few minutes to find the right location). A ten minute delay out of T3 and you are snookered. You best allow an hour to give contingency. Europe doesn't really do customs anymore, it's just passports you need to worry about.

And certainly not worth doing if this is just the town bus to BCN, I guess this is for further off.