Oh on which page is the bit about initiative units burning their initiative.
Opportunity Fire on pg 68 explains IU being able to use their activation to fire on enemy units that are acting or reacting.
A beats NIU1 and NIU2 but not NIU3 (is this 3 seperate rolls or just one roll each NIU throwing 1D)
Every unit will roll a separate reaction test against every unit reacting to it, like the reacting units will roll a separate TQ check to react.
It may mean that during the reaction resolution Unit A might lose to all three NIU's but the other two IU's beat the three NIU's thus pinning them or causing casualties before Unit A activates anyway.
With so many units activating and reacting at once it can get confusing.