Hi,
WAITSTR is used to wait for a specific string of characters and THEN continue.
In this case it'll sit there and receive charcters until three bytes in a row matches what you've stored in buffer and THEN continue with the program. Since you don't have anything in stored in buffer (all zero) you'd need to send three null bytes in order for the program continue.

If you want to receive three bytes, any three bytes, and store them in buffer then use STR Buffer \3 instead of WAITSTR Buffer \3. See section 2.10 in the PBP3 manual for more details.

/Henrik.