The Ghostbuster timeline simplified.

Okay, assuming for the sake of discussion that Ghostbusters 3 is a thing that's gonna happen, here 's how I think the timeline shakes out.

Ghostbusters 1 happens in 1984 in real time.
Ghostbusters 2 happens in 1989 in real time.
Ghostbusters The Video Game retroactively goes back to 1991 just after GB2.
The Ghostbusters IDW comics pick up after the video game, and happen in a vaguely defined mid 90's, somewhere between 1993 and 1996.

SO, if GB3 came out, it would definitely be present day, and unless they pulled a Disney/Lucasfim, the game and comics could still be kept canon in the 90's.

It's get a LITTLE trickier with Real Ghostbusters, and Extreme Ghostbusters, and Answer The Call.
ATC is 2016 in real time, RGB ran from 1986 to 1991 and there were signs and stuff in the show indicating it was the real year of the episode, so those were '86-'91 in real time, and EGB came out in '97, and it looked like the 90's, so assume '97 in real time.

BUT, IDW says those are all splinter timelines, and those splinter timelines have crossed into IDW-verse, so from the point of view of Prime Ghostbusters, it all squeezes into their version  of the 90's.

So...it's just...

• Ghostbusters (1984)
• Ghostbusters 2 (1989)
• Ghostbusters The Video Game (1991)
• Ghostbusters IDW comics (1993-1996) 
• Ghostbusters 3 (2020??) 

Where The Video Game and IDW Comics are one big giant Ghostbusters 2.5.

And cartoons, other video games, and ATC are part of that 2.5 as spin-off timelines that splice in at the crossovers.

But, if you wanna know how to organize 'em in your DVD shelf, I'd go....

• Ghostbusters
• Ghostbusters 2
• Ghostbusters 3
• Ghostbusters: Answer The Call
• The Real Ghostbusters

And the comics go...

• Ghostbusters: Displaced Aggression
• Ghostbusters: Infestation
• Ghostbusters: Spectral Shenanigans
• Ghostbusters: Who Ya Gonna Call?
• Ghostbusters: The New Ghostbusters
• Ghostbusters: Trains, Brains, and Ghostly Remains
• Ghostbusters: Happy Horror Days!
• Ghostbusters: Mass Hysteria Volume 1
• Ghostbusters: Mass Hysteria Volume 2
• TMNT/Ghostbusters
• Ghostbusters: Get Real
• Ghostbusters International Volume 1
• Ghostbusters International Volume 2
• Ghostbusters 101
• Ghostbusters: Answer The Call (What Dreams May Come)
• TMNT/Ghostbusters 2
• Ghostbusters: Crossing Over

And, there ya go.
The background text stuff in "Crossing Over", gives you even finer details.
But that's the broad strokes.
Enjoy!