The first two variants should be nearly equivalent in the big picture, assuming your investment strategy is the same in 3a and outside 3a. I prefer the second variant. It’s fairly simple and the same each month. And it works well also if you use a different investment strategy in 3a. The first variant might save you fees at your broker but it probably doesn’t make a big difference.
I would not follow the third approach. I assume you have a reason for the amount you keep in your emergency fund. What do you do if you have an emergency in the first quarter of the year? If you think you can handle emergencies just fine with 7k less, maybe you should rather reduce the size of your emergency fund.