The more the merrier! But yes, a minimum of three hens is usually advised.
You shouldn't have any trouble integrating them if you get your Marans hen on Thursday. It's unlikely your two will have settled in enough yet to be very territorial. If you do have any problems they should be fairly minor and you can find a lot of threads on here advising how to deal with any integration problems.
Am I correct in thinking the marans is coming from the same breeder? This would be ideal, as you won't need to quarantine to isolate any parasites or diseases.
Good luck, and have fun with your hens.
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