No difference EXCEPT that one is LARGE shank and the other is SMALL shank.
The Wick 3L fits a "Large" receiver -- i.e., a standard American or British receiver. This is true of any Wick mouthpiece with the 'L' designation. The Wick 3 fits the "small European" receiver -- which is very close to the same as a "large shank" trombone/euphonium/bass trombone receiver.
It's very likely that the tuba you mention has the smaller receiver (as does my 1924 Buescher tuba). So a Wick 3 (but NOT 3L) would fit it. However, my guess is that the 3 is quite likely too large for it -- that is, the mouthpiece interior size (bowl/rim) is too large. It definitely is too large for me on the Buescher. I used a Wick 5 on it for some time, but now am using a Kelly 25 (copy of the Bach 25) that I have modified by sanding down the shank to fit the receiver better. You might consider that since the 25 is probably a good size for the horn, and the Kelly is very inexpensive.
Most American tuba players feel that the 24AW is a TERRIBLE tuba mouthpiece (often referred to as a "paperweight" or "doorstop"). But it retains a big following in England. I won't go into the details since the web is full of discussions about this. However, I can't recommend that mouthpiece to anyone.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)