You need to understand the obstacles from your employers point of view and try to find a way to mitigate them.
You've said that your employer's reason was that you wouldn't get so involved in projects. That suggests that daily cover isn't needed - rather you're doing something like BA or project management where the time can be spread out more.
But still 2 days off in a row might mean too long a wait for something to progress. You might have more luck if you ask for non-consecutive days eg Tuesday and Thursday. If that's no go, then perhaps you could have the 2nd day off/week for a shorter time, maybe a month whilst you pick up your new projects, then down to 1 day off/week.
Condensed hours can work if you do substantial work completely by yourself (eg BA designs) but wouldn't work well if most of your job involves communicating with or coordinating other people (who wouldn't be around out of hours). If you can show eg 25% of my work is stuff you do alone, and that's what you'll do in the out-of-hours time, that might be acceptable.
Tbh, they're probably also worried that you just won't be as committed - and that doing 3 days/week you won't feel like you 'own' projects. That can happen, although you're just as likely to get almost as much work done in the shorter hours! Not sure how you can demonstrate that you'll still be committed: maybe highlighting where you've gone above and beyond in the past?
Btw, if you're trying to save the most amount of money, you'd do better to extend your maternity leave - or take the holiday as pay - since nursery costs more per day for 3 days rather than 5 (and 4 barely saves you anything) But of course saving money isn't the only thing.
It is tough going back to work. For a long time I felt I was just 'in the wrong place' when I was at work. But you'll be OK - and so will your baby. (and you'll love the uninterrupted lunch break!)