Bit of an improvement:
All three fly 75 miles, leaving them with 225 gals. P3 gives 75 gals each to P1 and P2, leaving them with 300 galls and himself 75. He heads home.
P1 and P2 fly a further 75 miles, leaving them with 225 gals and 150 miles from home. P2 transfers 75 galls to P1 leaving P1 full-up and himdelf just enough to get back for breakfast.
P1 can now fly a further 300 miles, making 450 in total.
You know what I did before I married Mrs Worrier? Anything I wanted to.