Do you mean sent as in shipped through the mail? The shipping process can reduce the odds of a good hatch quite a bit sometimes. I got 1 Saxony duckling out of 2 tries, of 8 total eggs. 3 total made it to the end, just the one actually hatched. In the first hatch I had local eggs, every one of those hatched without issue, so it wasn't the incubation that time.
How old is the spare egg? They only last for about 5-7 days, sometimes 10, depending on storage. When working with a small batch or shipped eggs, set everything you can. The eggs rarely do what you want them to. If you want 3 ducklings, specially working with shipped eggs, you may want to set a dozen. If there are extra, you can find homes for them.
In May I broke down and bought live ducklings and had them shipped. I ordered 10 and received 10 perfect little ducklings. So different than ordering 6 eggs and getting 1 duckling, or 24 chicken eggs and getting 3 chickens... I've had bad luck with shipped eggs.
Maybe look around for a larger incubator so that you can set more at a time? Staggering is workable. My first duck hatch was staggered badly... 1 egg here, 1 egg 2 days later... hatching went on for a week and a half. Or see if you can find babies in what you want.
