You can have more than one plant in a large pot, but not with any and all plants. In this case, it is not a good idea - this Hydrangea's height and spread is 1.5 to 2.5 metres with a root spread to match, so it's only one in a large pot I'm afraid. You could put three in now, but you would need to turn them out and pot them separately within 6 months to a year, otherwise you won't be able to separate the roots of each plant from the other, and they will become stunted because of lack of root room if left planted together. You don't say where you are, but unless you're in the southern hemisphere, these plants should by now have dropped their leaves for winter, as they're not evergreen.
If you do live somewhere that isn't experiencing winter currently, and you want instant effect, then you could use temporary flowering plants round the edges, with the hydrangea in the middle for this year. Things like summer bedding (lobelia, geranium, petunias, zinnias, that sort of thing) or, in winter, pansies or cyclamen, which will be removed within a few months anyway. After a year or so, the roots of the hydrangea should have spread sufficiently that you won't be able to plant anything else in the pot with it.