as far as i know 'tri-band' refers to 2, 5 and 60 Ghz
My question was about interferences. Are all these antennas (I mean the 8 ones in the asus router pic) directional ?
No, those are not directional, they're omnidirectional (all directions).
edit: to be more specific each individual antenna is omni but the antennas are probably used in pairs or 3 at a time with slight phase shifts in the signals to directionally shape the interference patterns. This is what "beamforming" is, but it's primarily a "far field" (ie. more than maybe 5 wavelengths away) phenomenon, and isn't relevant for near-field interference between the two or three radios that are essentially sitting on top of each other. At 5Ghz the wavelength is about 6 cm so 5 wavelengths is 35cm = 1.1ft = 14 inches, so if you draw a circle around the router that's a 1ft ~ 35cm radius then the beamforming is maybe pretty functional outside that circle and becomes somewhat more "sharp" as you get even farther away (say 1m or more).
There are two ways to avoid interference, one is to operate on widely spaced frequencies and thus be able to filter out the other radio with relatively simple band-pass filters and decent shielding. That's what @mk24 was talking about. So in these cases the 5Ghz radios may be say operating on channels 36 and 149 and the difference in frequency is 5745 - 5180 = 565 Mhz and therefore you can put analog filters on the circuits that keeps the upper band from showing up in the electronics for the lower band.
The other is to have very high quality shielding and tight frequency rejection outside your band, and thus do the same thing, except instead of filtering the whole upper band from the lower band you somehow filter out everything but the few tens of Mhz you're operating in. This is the more expensive way to go.
The other way is to just sell junk with improper shielding and then when it doesn't work all that well, at least you don't have any real competition and you've already collected the sucker's money and can move on to the next revision of your consumer junk
If you want multiple radios, you are by far better off buying separate APs and spreading them around your space on the end of longish cat6 wires: https://www.amazon.com/TP-Link-EAP225-V3-Wireless-Supports/dp/B0781YXFBT
With APs like this you can run the stock firmware to get full function/performance, they're relatively low risk, since they're not internet facing and they do support VLANs (I think!) so you can do all the management on a special VLAN.
edit2: @pamiller3 if you really are mainly interested in adding additional radio frequency capacity to your network, that's what I'd do, buy a dedicated AP like the one I linked to and put it on the end of at least 3 meters of cat6 so its spatially separated from your other radios... even better if you can put it somewhere farther away that you currently have lousy signal