Nick here from Shopify.
I'm wondering if the Locations feature will work for you here. Locations lets you set up multiple locations in your Shopify store so that you can track inventory and fulfill orders at your locations where different dropshippers can be different locations.
Once you have locations set up you can assign each location to a certain dropshipper and you can fulfill orders based on the location of your inventory. You can find out more about order management with locations from the help guide Shopify created about it here and the YouTube tutorial about it below:
Something to note with locations is that with each Shopify plan there are a certain amount of locations for each. You can see which plans have what number of locations from the screenshot:
Shopify has a very helpful guide on Locations which might be worth your time going through that you can find here.
Once you have locations set up, you can adjust the shipping costs in your shipping settings based on the different locations they are being shipped from.
Hopefully, this helps!
All the best, Nick