As with other elements of outdoor gear tents are divided into season categories, so what is meant by a 3 season tent?
A 3 season tent is one of the more common types to be used as it is designed to cover Spring, Summer and Autumn; the times of year when the vast majority of us are happy to spend time in the wilderness, sleeping under metaphorical canvas - thankfully materials have moved on slightly to provide lighter options!
The other tent you are likely to come across is a 4 season model, which despite its name is really mostly use din winter. Yes, of course it will do a job in other seasons but it is designed for the more dedicated adventurers willing to go out in the harshest conditions.
What are the key differences?
A 3 season tent is likely to have more ventilation, the inner, or tent body, having more mesh than would be found on a 4 season tent.
The poles maybe thinner and the tent body is likely to hook onto them. Furthermore the outer (rainfly or fly sheet) will not be fastened to the poles via sleeves (which would require the poles sliding into the rainfly for maximum stability).
This is an area where a 4 season would differ. The extra stability required for a 4 season tent means they are more likely to have this sleeve design for the poles and the rainfly would also potentially extend further. A 4 season tent is likely to have more guy points for attaching ropes as well as thicker and possibly more numerous poles.
Which tent do I need?
Now, of course, in reality there will be tents that are somewhere in between. Some ultra lightweight models offer the possibility to use hiking poles as well to stretch out the rainfly, further eliminating the need for the weight of tent poles.
As you can see from this brief description a 3 season tent is not designed to withstand snow, extreme cold and regular high winds. Its more down to weather conditions than actual seasons. You could easily use a 3 season tent year round if the conditions dictate it is suitable.