Don’t do what @MrCheese is suggesting, it’s a pretty bad idea. I would strongly advise to open up all the accounts you plan on having in the end and evenly contribute to them. That way you don’t run into problems of having way too much in one account in the end.
Let’s assume someone is 30 and will pay 30 years into 3a and plans to withdraw 5 seperate accounts starting from 60. Let’s assume that he/she is investing in Global100 (or whatever) and expects 6%/year in returns.
If this person would pay 6 years into the 1st, 6 years into the 2nd etc. the end balance with 60 would be quite a difference in each account. The first would have the most time to compound, the last almost no time to compound. If we would do it like that and pay 6800/year for 6 years into each account (one after the other), you would end up with the following amounts:
You end up wasting a lot of money due to paying more taxes. If you would just split it up into 5 accounts from the start, each one of them would end up with exactly the same amount, 114k each in my example (which is the same amount as those numbers combined devided by 5).