Need to create and i-rule which directs traffic coming from these 3 IP address to btsxv-old-pool and the rest of the traffic goes to btsxv-new-pool.

To expand on Shaun's answer, your data group should be of the type "Address", with the IP addresses entered as three records. If you do not specify a subnet mask for an address, it defaults to a /32.

Here is an example that I created showing a single host as well as a /24 sized subnet.

ltm data-group internal DG_SourceIP {
    partition Common
    records {
        1.1.1.1/32 { }
        2.2.2.0/24 { }
    }
    type ip
}