Here are a couple of suggestions, but as Benedikt pointed out, more info could help us a lot in answering you more accurately (your bandwidth speed (!) / home wifi setup / router model / etc.):
If you want everything to stream over WiFi, make sure you have a decent modern WiFi router. I would recommend something that supports 802.11n or higher, but make sure that your pc / laptop / tablet / phone also supports it.
If suitable, you might want to connect your computers (or at least 1 of them) through ethernet and check if you can stream multiple videos then. Even with a modern WiFi router, more devices generally means less bandwidth.
Make sure you are not too far from the WiFi router or have it setup in the worst possible location. You can use istumbler.net (mac) or stumbler.net (win) to check your signal quality.
If you suspect that your ISP is too slow (speedtest.net), you will have to see if they can either upgrade your plan or if this is all that is available for you. If you can't get a contract that offers above 3 Mbps, it might be difficult due to your location, but if you have a 15 Mbps plan, you shouldn't face any problems.
Check what quality you are trying to stream. If you are trying to stream HD content only, or even 4K content through netflix, you need up to 12 Mbps per stream, so you might want to find a provider with 50 Mbps or higher speeds.
Make sure you have a netflix account that allows more than one person to stream media (ok, this one should be trivial).
Lower your netflix streaming rate. In the Netflix settings you can adjust playback settings (Low / Medium / High). Try the lowest on all devices and see if that improves anything.
Lastly, if non of these things help you, maybe try to pre-load your stream. For example, if you intend to rent an iTunes movie, you can pre-download (cache) it at any speed you have available and watch it after it was cached to disk at any quality you choose (SD/HD). Obviously, this only works for this exact scenario (renting though iTunes) and Netflix doesn't offer such services, but maybe you find other similar offers through some Amazon player (I am not familiar with them, though).