You are in a very nice position here but you will have to research this all properly to make sure it all turns out well for you.
If I was you my primary concerns would be making sure you get a good price for what you do sell, but also that the buildings around your eventual home will be to your liking. If you get the PP for 3 separate houses and sell on that basis there is nothing to stop a developer from re-applying to get PP for apartments.
If you know anyone, or even a group of people that you know and trust it might be an ideal investment for them and you if you can agree that they finance it but it must be done to your liking (hey, they’ll be making a nice few quid either way). Failing that checking out several developers and seeing if you strike a chord with and like might be the only way for you to maintain control over the other plots. I know someone who might be interested if you are in the Dublin area so PM me if you want their details.
By the way, 350k is a lot for building a house so I’m assuming it’s a beauty!