Unless I missed something, you have single phone number and single SIP account. All three phones are using same account.
Well, then you have SIP equivalent of three standard analog phones connected to one standard analog phone wire in parallel. It's not possible to transfer/divert/deflect call from one line to another - just because all three lines are virtually same line, so there is nothing like "another line" here. You need either three independent phone numbers, or, you need own small switch in your office instead.
Hold on one phone then resume on another may be avaiable even with your current environment if supported by your telco operator. Ask them ...