three tables relation order is correct but values are false
It is common multiplying in multi-table source. Each record in table3 is joined to 3 (in above sample data, really it can differ, of course) records from table2.

If table3(t1_id,letter) is unique (by index) then COUNT(DISTINCT t3.letter) will help. 

If not then the only solution is in calculate this COUNT() in subquery by table3 separately and join the result to table1.