It is safe to plug all three in
The totalt switched power on your Denon 3808ci for the two outlets has a very low total capacity of only 1 amp (see page 22 of your owner's manual) . However you can safely connect the 3 Veris cooling fanas because their total power requirement is only 0.126 amperes.
The power of each Veris coling fan is only 5.04 watts which at 120 volts is only about 0.042 amperes. So it is fine to plug-in So all three of them is only .0.126 amperes. This is very low current and any extension cord even a small gauge lamp extension one would work.