Help with first time Gouldian breeders

[Steevee]

Hi. I am new to finch breeding. I have 2 pairs of young Gouldians who I believe have not bred before.
Around mid to late February, I put in 3 nesting boxes (each with a fistful of grass) in my aviary. I thought that nothing was happening until I found a smashed egg on the concrete floor of the aviary and another egg in the seed tray. Upon checking, the pairs had started laying in two of the nesting boxes. One pair have never sat on their eggs and today, I have discarded the eggs from that box. There were 3 eggs.
The other pair initially also ignored their eggs but then the hen started sitting periodically. This started more than a week after I noticed the eggs, so I am not sure if the eggs are viable. There were 3 eggs when I checked and I haven't checked again for fear of having the birds abandon the nest due to my interference. For the past 2 or 3 weeks, both parents have taken turns sitting on the eggs and are now constantly in the nest.
It seems that this pair now have the idea about incubating eggs, but I fear that it took too long for them before they started sitting and that the eggs are not viable. I remember reading somewhere that the eggs will only survive for about a week if they are not sat on?
It is now about 4 weeks since I first noticed the eggs. What should I do? Do I give the pair that are sitting a bit longer? Do I remove the 3 nesting boxes, give the birds a rest for a week or two and then put the nesting boxes back in? Any advice would be greatly appreciated. Thank you.

[Tiaris]

Check the nests. If still just eggs after more than a fortnight of incubating just chuck the eggs away. leave the boxes in there & they will renest when ready which should be fairly immediate at this time of year. They shouldn't leave the nest from you checking it occasionally.

[E Orix]

I have found that when the female Gouldian lays and her beak has not changed to blackish then success is rare.
Once the beak changes then they are generally on the job.

[finchbreeder]

Also just because you do not see them sitting does not mean they aren't. They have good hearing, and some pairs will leave the nest when they hear you coming, because they want to be ready to start on that lovely seeding grass you are bringing them. Also mostly the cock sits in the day and the hen at night. Good luck. PS I have a recent fledgling here from parents who were not seen to be sitting at all. Sneaky of them.
LML

[bazwang]

they will start incubating after the last egg has been laid

[Steevee]

Thank you all for your advice.
Both hens' beaks had darkened before I put the nesting boxes in, however, they could be a little bit darker now.
I can see the aviary through my kitchen window and for the past couple of weeks, every time that I look, there have been 3 finches visible. That's why I think that one pair never sat. That pair still only had 3 eggs after 3 weeks and I have now discarded those eggs.
I checked the nest of the sitting pair today and there are 6 eggs. Because I am not exactly sure of the timing, I will leave them a bit longer. If there are no signs of hatching then I will discard those eggs as well and hopefully they will start again . At least I have learned one valuable lesson - write down on the calendar the date they start laying.
Thanks again for everyone's advice.

[Tiaris]

After a week or so of incubation, fertile eggs take on a darker opaque chalky shade as opposed to clearer orange/pinkish tinted clear eggs.