Can I visit Thailand for three trips of less than 60 days each without a visa?

Yes

Thinking of spending 3 months every winter with my sis-in-law. Is this possible without a visa?

What dates? Thailand works off a calander year. You can have two visa free visits a year. Resets on Jan 1.

You will be totally fine don't let anyone tell you anything different coz you are doing things correctly!

The current guidelines suggest a max stay of 157 days using visa-exempt entries. There are no hard and fast rules, but I would be inclined to enter once visa-exempt and then get a METV

We used to do this as well, but lately we frequently faced problems/questions when we arrived. Next time we will apply for a multiple entry tourist visa.

Three separate stays of up to 60 days each within the same year, even with breaks of around two months in between, will put you near roughly 150–180 days total in Thailand within a rolling period. At that point, a third visa-exempt entry is no longer routine and becomes discretionary. In practice, it is closer to gambling than a predictable outcome. You may still be admitted, especially if your stays are clearly short-term tourism and you return to your country of domicile between trips, but you should expect a higher likelihood of questions and scrutiny. You must be prepared with a confirmed onward ticket, accommodation, and proof of funds, and you should accept that refusal is a real possibility. A tourist visa could give a slightly higher chance to get approved, but if you’re passed 50 years I would have applied for a Non O first.

How long are the trips ?

Were you here last year & if so how often ?