Conspiracy theory:

I hatched 3 Chansey in one day

Yes, I can confirm this has happened to me lots of times.

I hatched 3 Larvitar in a row from my last 4 eggs (before that had never hatched one). The 4th was a Mareep, I'd be glad if your theory is true and I happen to hatch 3 mareep in a row

The mon is determined at pickup I believe. Do you recall if you got those eggs close to each other both in time and area? When you got them the same mon was assigned. That’s how I think this happens.

you have to bear in mind that low probability events are remembered and reported more often then apparent randomness often leading people to see patterns where there are none.
You could be right but just take self reporting with a grain of salt.

I agree because I've gotten triples in 10k especially. Dratini and pineco in recent past. But I also have had 2k eggs mirror something I just caught a large number of times.