A healthy plant will fill every bit of that space, I promise you. However, most plantings are done much closer together. I'd give them 24 inches, at least.

There are 2 different kinds, I've had both the kind that makes a big bush (probably what you've got if it says 36") and it grows to about 5 feet tall. And there's a kind that has a more spreading habit, maybe 2 feet wide but stays very short - under a foot tall. Those could go closer together, like one every foot or so.

The bush kind are yellow/orange the lower kind are purple. That should give you a hint as to what you have.
Linda C

Hmmm, I believe they are the spreading kind. They are orange/red/yellow and the name is "Spreading Sunset". I planted them about 24" apart and hope they'll fill the space.

I have Lantana in my front hot sunny dry bed. It spreads to 5 feet and fills the area to the brim, Year after year. Give it lots of room.