Greetings...
good day,

I have a Mikrotik RB 750 , and I would like to configure it as follow :

from the first source the RB will receive only 40 Mbps on the first port, if the bandwidth more than 40 Mbps it will receive 40 Mbps on ether 1 and the remaining bandwidth will redirect to the next Ethernet port and so on for three ports , just like the image in the attachment .

best regards

You want your router to magically tell some source to start sending on a different link? You question really makes no sense.

you could use traffic monitor with scripts, and when load increases, enable some load balancing configuration.