Find/replace alpha characters in string

Hi,
I would like to find a 3-letter character series in a string/variable and replace it with x's.

An example set of strings is:

563MS333_101_afp_400-100_screening
563MS333_104-525_rjk_525_screening
563MS333_110_dlj_500-100_w24
563MS333_888-100_mmm_424_screening

The only constants are: 565MS333 and the time points at the end of each string (so 'screening', 'w24', 'w48', 'w96'). The rest can be variable.

I am trying to replace the 3-alpha characters in between the 3-digit numbers with x's (so, *_xxx_*). The 3 alpha characters are usually between underscores, but not always.

Thanks for the help/input!