why i can not connect the 3rd display to my mac mini

What Mac Mini do you have? Find out by going to the Apple Menu > About This Mac. Mac Mini (M1, 2020), supports two external displays, as listed clearly on Apple’s website.

You can connect a third by using an iPad via Sidecar, or by using a third party DisplayLink capable dock (this creates a software driven display, that is CPU driven.)

It is possible to run three monitors with a 16 GB M1 Mac mini.  Here is my set up in a flight simulator with OS Monterey. It is connected to three monitors, all of which work, as follows: 

(1) To a Dell model 2007 FPS monitor (1600 x 1200 @ 60 Hz via VGA on monitor to VGA/DVI-I adapter to Startech.com USB 3.0 Dual Head Graphics Adapter to USB-C on Mac Mini).  DisplayLink driver from startech.com was loaded and activated.

(2) to a Westinghouse TV model LDD2240 (1920 x 1080 @ 60 Hz via HDMI to HDMI).

(3) to a Flatron W2252TQ (VGA ơn monitor to Belkin VGA/Thunderbolt adapter to Thunderbolt on Mac Mini). 

The three monitors do not all start up simultaneously, but all three come on in about a minute, with no further action on my part.  It is as though they need to figure who is the boss.