Based on my experience (girl 12, boy 5, boy 3) random things to consider before adding another:
The age gap will result in dividing the family on many outings - the older ones and the little ones. Each group will need a parent at least for a while.
Little ones are a lot of work and after you have left baby/toddler mode behind it, can be difficult to want to get back into that phase.
Older ones spoil you to the “grab shoes and run out the door” lifestyle which is most closely like life before kids. Little ones will drag everyone back to a much slower pace with more planning involved.
Three kids may form factions. In our house the oldest and the littlest mesh like peas and carrots naturally. It leaves the middle guy out a lot. Before with only two it was never an issue. If they didn't want to be around each other, then they were alone. Now there is one loner by choice or default and that is not the same dynamic.
The kids may not like each other and they may not get along very well. This may be a phase or it may be a fact of life. Our kids get along for the most part but our friends with three have two who simply cannot stand each other. They spend a lot of time dividing the family to avoid the negative interactions of the two who don’t get along.
The kids may not be thrilled there is an addition (read competition for resources, attention from parents, change in outings, change in dynamics, etc.).
Older ones can’t always relate to/handle little ones so the impatience factor can be high which drives up fighting, ignoring each other, nagging each other, etc.
The division of watching kids is no longer one parent-one kid.
Finding help to watch three for a date night is not the same as finding help to watch two.