She may be getting ready to go broody, when she feels she has enough eggs to sit on. I don't know when they decide enough is enough to stop laying and start brooding, but three eggs is probably not enough.
She may not actually be 'hiding' them....she just found place to lay she likes better than the coop nests.
Free range birds sometimes need to be 'trained'(or re-trained) to lay in the coop nests, especially new layers. Leaving them locked in the coop/run for a week or so can help 'home' them to lay in the coop nests. Fake eggs/golf balls in the nests can help 'show' them were to lay. They can be confined to coop and maybe run 24/7 for a few days to a week, provided you have adequate space and ventilation, or confine them at least until mid to late afternoon. You help them create a new habit and they will usually stick with it. ..at least for a good while, then repeat as necessary.