4 in. x 4 in. x 4 in. Wye

I would NOT think it would be a good idea to run two dryers through the wye. With no shutoff in either duct branch, a significant quantity of pressurized dryer (assuming it's gas) exhaust fumes would backup into the house through the idle dryer. I contemplated a similar use, but not for two dryers but with one dryer and a vent for a portable air conditioner - same problem. I bought two manual dampers (1 for each branch), but they leave an approximate 3/16" gap around the entire damper plate, so still a problem. Plus the need to remember to manually change them back depending on which appliance was being used.There is a 4" spring loaded one-way damper that is used for bathroom venting, but while it shuts automatically if there is no airflow, it seals with a rubber gasket that I fear would not hold up to dryer exhaust temperatures. The risk of CO poisoning was just too great for me to consider either option.

Yes i used it to con ect two dryers to a single exhaust

Just like the picture, if you need to connect three 4" ducts you'll have to purchase a 4" x 4" duct connector (both ends male) that it appears Home Depot has discontinued.

I have to vent two in one it was the best to find something like this save me work and time

Yes it is tapered as shown on the picture for for 4" duct on the upper outlets. The picture is deceiving because it looks like the bottom connection is larger that 4". Actually it is 4". I slid on 4" flexible ducting on all three ends without an issue. I used zip ties and a couple of metal screws to secure the ducting.

Physically it can, but those should probably be on separate exhaust lines. Not typical to combine exhaust like that.

I believe you could vent both fans but you would need to install a spring-loaded butterfly valve between them in order to prevent blow-back from the exit of one fan to the opening of the other.

Yes, but you would need to install a spring-loaded butterfly valve to prevent the blow-back from the exit of one fan to the opening of the other.

I used it for something similar (bathroom & air scrubber). You will want gates to prevent back flow - nothing like bathroom odors blowing over your cook top.

All 4"