Check that your filter for CW TX is correctly set (either a 2.7 or 2.8), and if you have a 2.7, it should have an offset like -.89 If a 2.8, then zero offset.
Set PWR SET to nor, then set power to 11.9 Watts (7.9 on 6m).
Then long press XMIT to do a TUNE and see if it will make good power on all bands (I assume 6m is dead).
If KXV3 equipped, put a dummy load on the XVTR BNC and runt he MW tx gain cal, that can show problems.
If the MW passes with good numbers (please send), and the LPA does not make good power, the LPA has an issue.
6m problems can be one (or more) of the 4 little blue coils on the mixer board L1-L4. The should be about 4 ohms and can easily be damaged and open.
Elecraft K3 Tech.