Sorry but flexi days are not about going out and eating what you want without counting.
If you know you have a big event coming up you plan in advance how many extra syns you are going to allow yourself whether that is 30, 40, 50 or whatever.
On the day you continue to follow your SW plan and counting your syns. When you reach the limit you set - you stop.
The next day you get back onto the normal plan, usual 5-15 syns. This is important, you shouldn't cut back in an attempt to pull back your week. You may not lose weight that week but the idea is that you have remained in control of what you are eating and can continue the next day without feeling guilty.
You can't do it retrospectively, have a blow out one night and decide afterwards to call it a flexi day. That defeats the purpose because you were not in control.
And it is only supposed to be once in while, not a regular thing and def not three times in a week