Let's see:
Day 1:
Worthing - London Terminals SDR £27 adult £13.50 child
OR Worthing - Boundary Zone 6 £21.20 adult £10.60 child
Z1-9 Travelcard (buy from Bushey if buying online or from a NR ticket office) £19.60 adult £9.80 child
Day 2:
Z1-9 Travelcard £19.60 adult £9.80 child
Day 3:
Z1-9 Travelcard £19.60 adult £9.80 child
Boundary Zone 6 - Worthing £20.40 adult £10.20 child
OR London Terminals - Worthing £25.50 adult £12.75 child
As you can see buying from Boundary Zone 6 will make things cheaper although you will need to go to a booking office to get them.
Also if you are arriving in London on Day 1 off-peak and leaving on Day 3 off-peak you could get an Off-Peak Return (SVR) to use instead of 2 singles, saving more money