When you remove a member during the billing period, we will not charge you the full price for this user. We'll add a prorated amount in credits to your account as a refund for the time you pre-paid for but did not use.
You are in the Team plan and have 3 users in your team. This means you are paying $18 x 3 = $54 monthly.
Say, you decide to remove one member in the middle of your billing period. In this case, we will refund a half of the cost you paid for the user, i.e. you will receive $18 / 2 = $9 in credits. In other words, next month you will pay $9 less.
This means you will get a $9 discount on your next month's payment.