Arrow 3x03 should take place before The Flash 1x04. Because the last scene was hinting at the mini crossover. Felicity was learning that his friend has woken up from coma and was going to visit him, and then she was in The Flash 1x04.
Adventures of Supergirl:
There's a week between #3 and #4.
#7 takes place a week after Supergirl 1x06 and three weeks after #4
#7, #8, and #9 take place in the same day
In Supergirl 2x11, Kara mentions of her Earth birthday, which was in December 15. And Supergirl 2x09 was about a week after Invasion! So 2x09-2x11 should probably be in between Invasion! and The Flash 3x09 in the timeline.
Chronicles of Cisco Post 50 takes place before Flash 3x12's last scene. In the post, Cisco talks about him and Wally "pining for their other-Earth baes" (Gypsy and Jesse). And as Jesse's return occurs in 3x12's last scene, the post has to take place before that.
The Flash 4x02 needs to take place before Arrow 6x02. An article in The Flash 4x02 says "Mayor Queen is the Green Arrow". This was claimed at the end of Arrow 6x01, and Oliver got clean at Arrow 6x02.
It's very likely that Legends 3x01 is before Flash 4x01. Because Wally and Nate's dialogue implied that Wally was still The Flash and that Barry hadn't came back yet.
In The Flash 5x04, it is said that the memes about Ralph (from 5x03) were from "a few weeks ago". And in 5x17, the entry about Spencer Young (who appeared in 5x04) is specifically dated as May 31, 2018.
There's a two-week gap between The Flash 5x04 and 5x05. And, Chronicles of Cisco post 86 takes place after 5x05.
There's a week in between The Flash 5x05 and 5x06.