What can I use as a more modern equivalent to a AF3?
there is no direct modern equivalent, the AF3 was the end of the line.
However, you can use a 6K7 (-G, -GT) as substitute, by changing the socket and supply the 6.3 V heater voltage, probably by a small 4 to 6.3 V step-up auto-transformer or by some additional windings on the power transformer. As well you need to realign the surrounding tuned circuits.
A 78 may also work.
Best Regards, Jacob