Tricky. You cannot do it the way you envisage: apply your Bold style to three items in a row but not to the stuff between them. You can apply it selectively to (uppercase) followed-by “digits uppercase digits uppercase digits”, but that only works for the first item. To use this same trick for (uppercase digits) APPLY_HERE (digits uppercase digits), you need a variable-length lookbehind because there may be one, two, or three digits after that first uppercase. Of course you can add a GREP style for each of the three possibilities; but then you have to add, uh, (3 x 2) six more possibilities for the third part.
So I propose the reverse: apply bold to the entire string of uppercase/3 digits; then, apply Regular again to just the digits. (Oh–and I see a single '+' in there as well!) That would be
1. Bold to
bud{1,3} ud{1,3}+? ud{1,3}b
2. Regular to
(?<=bu)d{1,3}+?s
Note that this works on the small sample you provided, but may have side effects if there are any more uppercase-digit combinations in your text.
It's simpler if you can be sure it's just these three 'L', 'D', and 'H' (followed by one to three digits) that should be bolded, and they do not occur more than three times (i.e., never something like “L10 H20 D30 L40” — but guessing what they stand for, I think it's safe to assume that):
b[LHD]d{1,3}b
If this line is always a separate paragraph, consider creating a paragraph style specifically for it.
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