For Wimbledon, I did the daily queue back in 2016. I showed up at 6 AM on one of the weekdays in the first week and waited in a long, long line to get tickets onsite. 6 hours later, I was watching matches on court 8 from the 2nd row. I'm sure there are other ways to get tickets ahead of time, but my point is that it's possible to get in even on the same day, and I'm sure that's still the case given Wimbledon's traditions.
For the French Open, I understand that they don't sell tickets or grounds passes at the tournament grounds. Everything is sold online. And they have a somewhat complicated lottery system to buy the tickets ahead of time. The French is the one Slam I haven't been to yet. I went to Roland Garros to visit the site once, but not when the tournament was happening. I'm sure you'll be able to figure out how to get tickets online, or maybe someone else here can elaborate on their experiences.
I did the US Open in 2006, and bought tickets from the USTA (via TicketMaster) a few months ahead of time. The tickets were reasonable back then, but have gone up considerably now.
My favorite Slam I've attended has been the Australian Open. It feels like all of Australia loves tennis and everything is geared towards the tournament during that time. The tickets are relatively inexpensive and easy to get online, even on the same day.