Charges never post on the date they are made but when they do post a couple of days later they usually do post with the date of actual purchase. But it's also possible that the date it will post with will be a later date, I just don't think it will be as late as 2 days later. Usually I've seen it be a day later at most. It depends on the merchant and how/when they process and close out transactions.

As an example, on June 30 I made a Chase Edit booking with a new CSR Biz card to quickly try and get the $250 1st half year credit. The email confirmation said 6/30, pending charge said 6/30 but it posted 2-3 days later with 7/1 date and the $250 credit came out of the second half year.

By contrast, I made an online order last week on 10/17 with a merchant that I had an offer with and that offer had an expiration date of 10/17. I was not very confident at all that I'd get the credit for the offer given the risk that I am very well aware of when using offers on the very least day, but was placing the order anyway so figured it was worth a shot using that card in the off chance it would trigger the credit. Lo and behold, the charge posted on 10/20 with the 10/17 date and the credit from the offer posted to my account as well.

TLDR: I think it's worth a shot to put that large charge through to get 3x points but be cognizant of the fact that it's a risk.