If your PC operating systems support it, you could set up ingress policing and egress traffic shaping on each PC, so that each cooperatively limits itself to 1/3 of the maximum download and upload rates of your ADSL, respectively.
For example, in Linux, the following from this blog uses ingress policing to limit incoming traffic to 256 kbps:
tc qdisc add dev eth0 handle ffff: ingress
tc filter add dev eth0 parent ffff: protocol ip prio 50 \
u32 match ip src 0.0.0.0/0 police rate 256kbit \
burst 10k drop flowid :1
The following from the same blog uses the "Token Bucket Filter" (tbf) queuing discipline to limit outgoing traffic to 256 kbps:
tc qdisc add dev eth0 root tbf \
rate 256kbit latency 25ms burst 10k
There is a lot more information about traffic policing, shaping, and routing in the Linux Advanced Routing & Traffic Control Howto.