This is both procedurally and mathematically incorrect.
[...]and it actually hurts Tizona
It doesn't. Neither DA nor TA will ever actually *reduce* your DPS with AM3.
It reduces TA rate and stacking STP over DA gear for AM3 is optimal if cant stack TA but it doesnt hurt DPS just TA rate. Agreed?
This is also incorrect.
OAX is checked last when calculating multihits. The order is QA>TA>DA>OAX. As such, AM3 cannot reduce your proc rate for any of the preceding multiattack stats.
Furthermore, even DA is always a net gain over AM3. Look at it this way: for any hit where DA procs, it will add one hit. If you remove that DA proc, AM3 will on average add 0.8 hits (40% OA2, 20% OA3). This is a very small change in multiattack rate, but BLU generally dual wields so you also have a non-OAX offhand to account for.