Reddit reviews DynastyMattress King Bed 8" Thick Wood Box Foundation for Memory Foam, Latex, and air mattresses

https://www.amazon.com/DynastyMattress-Thick-Foundation-memory-mattresses/dp/B002RVCSRI/

If that is the one you ordered, then good luck, you will need it. I see that 16% of the reviewers give it only 1- or 2-stars, and most of those cite problems with poor wood quality (cracked, broken, warped) or with the staples and nails holding it together having come out. See this for example:

http://imgur.com/a/dZpii

But maybe you will get lucky and receive one without cracked wood or broken parts. If so, I bet you that the space between the slats will be more than 3 inches (ignore the morons on amazon who responded to questions about the slat spacing -- their answers range from 1 inch to 6 inches). My guess is it will be about 4-inch spacing with slats a little less than 2.5-inch wide (probably 13 or 14 "slats" total, counting the boards at the head and foot of the frame...you generally want to see 16 or more slats to get the spacing below 3-inch).

You could probably add some slats if you can do some basic woodworking (cutting boards, drilling pilot holes, inserting wood screws). Home Depot or Lowes have 8-ft long 1x3 furring strip boards (actual width is about 2.375-in) for a couple bucks. If you do this, be sure to drill pilot holes before screwing into the wood. With this low-quality wood, you risk splitting the wood if you are not careful. Even with pilot holes, if you don't have a counter-sink bit, you could crack the wood when the bottom of the head of the flat-head screw is driven into the surface of the wood. Maybe you can avoid screws and just attach extra slats to the straps with a staple gun, or you could cut small rectangles of wood from the furring strip cutoffs and use wood glue to attach them to the foundation rails so that they make a guide to hold the slats in place.