Since he is smaller, Ruby may not see him as much of a threat. She may be excited to have another high engery dog to run with!

One thing we have done with my dogs, is give them each thier "alone time"...I let just one go with me to the store, or do a therpay visits, or even just stay in the house and play with me while the other two go out. Or Ralph may take one somehwere, while I have the other two. It may help, if every couple of days you can put Zeus in another room to rest, and play with just your two, or leave Zeus home while you take Captain and Ruby for a walk...something to let them know you are not neglecting them, and give them a feeling normalacy for a time period.