This is a very odd setup, normally you do not have 3 "modems" each with DHCP running. Unless you have 3 different networks or have a good network guy that configured things for you. In which case your question should be directed to whoever setup up this network. Also the question "will it affect the whole network", all depends on what you mean? Will the network go down? Will it slow down? Will some things work and some things not? Will some computers have issues? Everything depends on the exact setup.i have 3 modem using dhcp setting in one network. when one modem down, why will affect whole network?