I've been told that the "3" at the start of his record number on these forms indicates that he enlisted in the North West of England.
If you're referring to the Regional no. 3/A this is the office that dealt with pensions for that region.
In this case 3=North West (office at Duke St. Mancs.)
A=Airman.
25 TS was at Thetford Norfolk.
Following the Easter uprising in 1916 it was decided not to include Ireland in conscription.
This didn't prevent men from enlisting if they so chose.
So that just leaves 43 Stn.
The only suggestion I can offer is that there was a Training School based in Reading for pilot/observer training although I can find no reference to it being called 43 Stn.
It may be he was posted there in some sort of support role.