A 3 person sauna is the size people land on when two feels tight and four feels indulgent. It is a real step up rather than a marketing one, because at three seats the bench layout stops being a single run and starts being an L, which changes both how the cabin is used and what it costs to heat.
What three seats buys you
The practical gain is not a third person. It is that an L shaped bench lets one person lie down fully while another sits, and that a family of three fits without anyone perching. If you mostly use a sauna alone but occasionally with a partner and a child, this is the size that stops being a compromise.
The bench layout question
Three seats can be one long bench or a corner. A long bench is better for lying down and worse for conversation. A corner seats three comfortably and rarely lets anyone stretch out. Makers show both and describe both as three person, so look at the plan view and decide which use you actually have in mind.
Power steps up here
At this volume a traditional heater usually moves past what a standard circuit will carry, so you are into a dedicated higher current run and an electrician. Infrared cabins at three seats often still sit on a normal circuit but not always. This is the size where the electrical cost stops being an afterthought, so price it before you order.
Heat-up and running cost
More air and more timber means a longer heat-up and more energy per session than a two seat cabin, on the order of a third to a half more for a traditional unit. If you will usually be in it alone, that is a recurring cost for capacity you are not using, which is the honest argument for staying at two.
Questions people ask about 3 person sauna
Is three person much bigger than two?
In floor area yes, and the bench becomes an L in most designs. That is the real difference rather than the seat count.
Will it run on a normal circuit?
A traditional heater at this size usually will not. Infrared often still does. Check the stated amperage before you commit.
Can one person lie down in it?
In a long bench layout yes. In a corner layout usually not, so choose the layout for how you will actually use it.