One option is to attach a 11g airport etreme station to the third computer by LAN port and have that be the main base station. this would make the whole network 11g. or merely upgrade the 3rd computer. However apple states that 11b is the standard for airtunes and their literature suggests trouble shooting by switching all components to 11b. also some users have found that using additional base stations interferes with airport express rather than augmenting the network as advertised. You should however be able to tell if the upgrade would change anything by temporarily switching to 11g and turning off any 11b devices in the network. the channel switch has helped my situation for a day or so now. good luck
Dimitri
Dimitri