Also, have you tried this:
Roland Knowledgebase Article # 103310
Product: TD-3 V-COMPACT-SET
The volume, or “level,” can be adjusted for any of the
instruments in a drum kit. Use the following steps to
adjust the level of an instrument:
1. Select a drum kit.
2. Press LEVEL/PAN so the level value is in the
display — the LEVEL/PAN button flashes.
Note: Each press of the LEVEL/PAN button
alternates between the level and pan settings.
3. Hit the pad to select the instrument whose
volume you’d like to change.
4. Press the + or – buttons to increase or decrease
the instrument’s volume.
5. Press DRUM KIT when you’re finished. The
settings are stored automatically.