Yes that's right but that is why I go on and on about not working Mondays (if that is possible) if your employer works like this (as most do).
Actually 5 of the 8 bank holidays are fixed and NEVER move. There are ALWAYS 4 bank holidays which fall on a Monday (Easter Monday, 2 x May bank holidays, August bank holiday), 1 bank holiday ALWAYS falls on a Friday (Good Friday). The other 3 (Christmas Day, Boxing Day, New Years Day) are the only3 that move.
I did exactly the same as you - 3 days - had 5 bank hols added to my annual leave and had to take a day's holiday if a bank holiday fell on my normal working day.
In 2016, the 8 bank holidays are 5 Mondays, 1 Tuesday and 2 Fridays. You will therefore have to use the 5 days you get for bank holidays, plus 1 of your own annual leave days just to cover the bank holidays. (If you didn't work Mondays, you'd use 3 of the 5 bank holiday days, have 2 left over, plus all of your own annual leave. Working a 3 day week, this equates to an extra week off (3 days)).
2017 is even worse - there are 6 Mondays, 1 Tuesday and 1 Friday. It will therefore cost you the 5 bank holiday days, plus 2 of your own annual leave days. (If you didn't work Mondays, it would cost you 2 of the bank holiday days, so you'd have 3 to spare, plus all your own annual leave - when you're only working a 3 day week, this is actually an extra 5 days so almost 2 weeks).
In 2018, there are 5 Mondays, 1 Tuesday and 1 Wednesday and 1 Friday. Again, it'll cost you 2 days of your own annual leave, plus the 5 bank holiday days.
You only get 5 days for bank holidays, and you will use 4 every year for Mondays. The only way you gain is if there are no other bank holidays at all throughout the year on Monday, Tuesday or Wednesday. I've looked at bank holidays upto and including 2024 and there are no years at all where there are only 5 bank holidays that fall Mon-Wed.
If at all possible, can you have a chat with your employer to see if you can work Tues-Thurs or Wed-Fri?