If all three devices (ipad, andriod and laptop) were say watching a video like BBC iPlayer at the same time it could indeed slow things down.
However for normal use I doubt anyone would notice the difference.
And only devices using the internet would have an effect, if the devices are off, or on but not doing aything on the internet then it would make no diference.
However I have noticed the internet has been a little slower over Xmas as I assume more people are at home and using their computers and other internet devices, so it is slowing the internet down for us all.
p.s. if your internet speed is very slow most of the time then multiple devices will make more of a differance than if it was a faster broadband. I have 60mbps with Virgin and it can cope with multiple devices because it is a fast broadband connection.