My first thought is that all three can get quite big and 6x4 isnt big enough for all of them. I think I would choose only one of them and surround it with smaller plants , bulbs, lower growing shrubs, etc.

Agree with Inverglen - at that size, an Amelanchier should be put smack in the middle, and only surrounded by much smaller shrubs/plants than Choisya, which gets 5 x 5 feet high and wide.

And it depends how far the bed is from the house too. The amelanchier will get quite tall, 3 - 10 metres depending on species and how long you give it to grow.

My Choisya did not do well in the heavy clay in my garden - they do not like to have wet roots when it is wet and cold in the winter (I had sanded and prepared the planting area and bark mulched to no avail)

To echo the others, if you like Magnolia (there is much to like) it will get big. If you can get a large pot to bury the magnolia in that will prevent it getting too big.

thank you all for the thoughts

why not have a water feacture instead . just a thaught .

If you really want a magnolia what about magnolia stellata - that doesn't get big. But I agree with the others, you only have room for one of the shrubs in a bed that size.