The Trixie comprises of 4 bets
3 x doubles and 1 x treble
You need at least 2 of your selections to be correct to get any returns.
If you place a £1 trixie bet that will cost you £4
What I tend to do is write the selections and then write the bet as
3 X £1 doubles
1 x £2 treble
which only costs an extra £1 but if you do get all 3 selections correct increases the payout significantly