£50 on Arsenal pre-game with Bet365 @ 11/8 returns £118.75 (+£50 will be refunded on the in play bet)
£50 on the draw in play with Bet365 @ 12/5 returns £170
£30 on Chelsea pre-game w/ William Hill returns £77.50 (+£50 will be refunded on the in play bet)

£130 out.
Profit breakdown:

Arsenal win - £32.75
Draw - £40
Chelsea win - £13

Or if you think Arsenal won't win, can do this:

£50 on Chelsea pre-game with Bet365 - 23/10 - returns £165 (+£50 refunded)
£50 on draw inplay with Bet365 - 12/5 - returns £170
£40 on Arsenal pre-game with SkyBet - 11/8 - returns £95 (+£50 refunded)

£140 out.
Profit:

Chelsea win = £75
Draw = £30
Arsenal = £5