Apparently you don't know why the first two you put out there had the problems, that makes it harder. Hopefully you worked with some people on here to try to determine what was wrong. I don't know if it was some disease or something about your facilities or something else Here in the States we can take our dead birds in for necropsy (autopsy) to determine what killed them, sometimes for free and sometimes for a cost. Each state is different. You might check with your agricultural ministry or whatever you call it and see if that is available for you in case it happens again. Hopefully they have a local office.
I'll just look at your basic integration issue. It would help a lot to know what your coop and run look like as well as a physical size in either feet or meters. Photos can be really instructive. But you have three chicks currently 9 weeks old and are looking at bringing two POL pullets in pretty soon. The 9-week-olds are plenty old enough to go outside without any supplemental heat.
I don't know how much you know about quarantine and why it is used. If often takes a few weeks for animals that have recently been exposed to a disease to come down with symptoms. if animals have not been exposed to other animals recently and the person that has been keeping them would recognize a disease if they saw it, they have essentially been in quarantine. Those are two big if's. You have to trust someone who is probably a stranger, that can be hard to do. Some flocks can develop flock immunities to certain things. Cocci is a great example but there are others. No matter how long you quarantine them they will not show symptoms but they can be carriers and infect other chickens. That can be your flock infecting new-comers as easily as new chickens bringing in something. That can be parasites as well as diseases. It's not a bad idea to at least inspect for parasites and actually treating them for mites, lice or worms before they mingle with your flock could help. I don't know where you got your first chickens or the three you have now. They may have brought something in.
Parasites and diseases can be transmitted by physical contact, eating or drinking from the same sources, eating dirt that others have pooped in, by insects or other animals like mosquitoes, or just blow on the air. Housing them side by side is not going to do much. The more physical separation you can get the better the quarantine. Changing shoes between flocks and using different buckets and storage for feed and water can help you not carry disease or parasites from one group to the other. A lot of us are not physically set up where we can achieve this or are not willing to do the work involved. Whether or not you quarantine and how has to be your decision.
One of the big issues with integration as you propose is the big maturity difference between your three and the POL pullets. That adds a complexity that you can avoid. Integrating pullets or hens at the same maturity level should not be a big deal as long as you have sufficient room. There are tricks we can use to smooth that process but if they have a lot of room many old-timers just turn them loose together and let them work it out. They usually do with little drama. But I'm not talking about that 4 and 10 square feet per chicken coop and run thing you often see on here, I'm talking about real room. But if you don't have that room some of the tricks are to house then side by side for a week or more where they can see each other but cannot touch, separate feeding and watering stations, and improving the quality of what room you have by giving them places to hide under, behind, or above. If we knew what your facilities look like we might be able to offer specific suggestions.
My option 3 for you would be to get two more pullets the same age as your current ones, option 4 would be to wait until your pullets are POL and get some the same age then. Go through a regular integration. My preference by far is option 3, get them now. That way you don't have isolate one group from the nests you want them to lay in when they are starting to lay.
Good luck!
