All three, and they are worth separating, because they behave in completely different ways and only one of them is hard.
Place: ที่ is 'at', and in front of a noun it makes the place of that thing. Relative: ที่ opens a clause that describes the noun before it. Ordinal: ที่ before a number makes it an ordinal, which is also where dates come from.
- ที่ไหนtʰîː nǎj
where
the place one
- คนที่ไปkʰon tʰîː paj
the person who went
the relative one
- ที่สองtʰîː sɔ̌ːŋ
second
the ordinal one
The relative one is the whole prize
French makes you choose: qui, que, où, dont, lequel, and the choice depends on the grammatical role of the thing being described. Thai has no such choice. Everything is ที่, and after it you say the clause in ordinary word order with the shared noun simply missing.
This is the point in the course where your sentences can suddenly get long, and it costs one word.
- ร้านที่ผมชอบráːn tʰîː pʰǒm tɕʰɔ̂ːp
the shop I like
- คนที่อยู่ที่นี่kʰon tʰîː jùː tʰîː.nîː
the people who live here
both ที่ in one phrase, doing different jobs
And the one you will read but never say
ซึ่ง does the same relative job in written register. Newspapers, contracts and signage are full of it and conversation has almost none.
Recognise it, do not produce it. Using ซึ่ง in a market is the register error that goes the other way, and it is much more conspicuous than being too casual.
- ซึ่งsɯ̂ŋ
which, that (written register)