If you had been on a 3 day week from the start, you would have 4 working weeks, i.e 12 days paid annual holiday. May to December on 3 day week will give you 8 days entitlement.
You worked 5 days per week for the first 3 months, entitling you to 5 days paid leave. If you will have worked over 117 hours in April you will have another 1 and 2/3 days.
So at the moment I estimate you can take 14 and 2/3 days, but if your working week increases again, or work at least 1365 hours in the year (which includes holiday time, overtime, and public holiday hours) you could find your entitlement goes back to 20 days.