Multi city option is there and works fine but 2 segments only, not three as you are proposing
So for example
DUB-ORD-SEA,
BWI-BOS-DUB
Thats possible in the website
So you can book (assuming valid connections)
LAX-DUB
VCE-DUB-LAX
And then a separate DUB-VCE
Anything fancier like forcing a multi day layover you need to call an agent, it is possible and is likely cheaper to call an agent