Outgoing port numbers in general are basically extremely unpredictable. Port numbers are only strictly applicable to TCP and UDP, but ICMP "identifier" fields can work in the same way as regards NAT.
There are a few protocols where the outgoing call has to come from a particular port number, but the most common operating systems's mplementations choose ports from a range for this purpose.
RFC 6335 covers this:
the System Ports, also known as the Well Known Ports, from 0-1023 (assigned by IANA), the User Ports, also known as the Registered Ports, from 1024-49151 (assigned by IANA), the Dynamic Ports, also known as the Private or Ephemeral Ports, from 49152-65535 (never assigned)
Some operating systems choose the ports randomly from a range, some choose them in linear order (like you see in your example). Some choose lowest available number from a range. It's entirely up to the operating system. Some operating systems let you choose the ranges. (A system with a great deal of outgoing connections, perhaps a web proxy, a mail gateway, or a NAT device, might run out of port numbers if the range is too small.)
In common speaking, many people just say some random high port number.
In your case, the WAN-side port numbers are chosen by the NAT devices. The LAN-side port numbers are chosen by the three hosts .1, .2 and .3, but note that it would be extraordinarily unusually for three hosts to choose the idential two port numbers for outgoing connections without something enforcing it. (Running in sync is unlikely to do it -- a transient failure with a retry, for example, will "use up" a port number in whatever sequence the OS is using.)
As Eddie says:
A lot of Cisco router's will choose Source Ports sequentially. A lot of F5 devices choose Source ports randomly. Every vendor is free to do it their own way.
A point to notice is that if you are planning access control on port numbers, you might have to accomodate whatever your operating systems use.
If for some reason you need to control particular port numbers for outgoing NAT, you need "static PAT" in your NAT device.