How to combine three table to find Avg,High,Low value in mysql?
try use union all + avg + max + min

select avg(`net_wpm`) `avg`,max(`net_wpm`) High,min(`net_wpm`) Low from (
  select * from prac_test union all
  select * from multi_test union all
  select * from shadow_test
) T
group by `UID`


Demo Link

Result:

| UID |     avg | High | Low |
|-----|---------|------|-----|
|   5 | 65.8333 |   97 |   8 |