We often hear the expression in gardening to plant in 3's or 5's. This is also true of a large pond area or where you are using tall marginal pond plants.
Place 3x 1 litre baskets of tall pond plants into a mesh contour basket and fill the remaining space within the contour basket with gravel.
This will achieve 2 things - firstly it will quickly look like a mature group in your pond as the 3 plants grow together to make a clump and secondly if the group is of a tall specimen of plant it will put weight at the base of the group enabling the taller pond plants to remain more stable when placed into the water and stop them blowing over in the wind.
Avoid the idea of mixing 3 different plants in one contour basket as this can look bitty and the more vigorous will always grow quicker and swamp the less vigorous. If you do put 3 different plant species in one basket they all need to like a similar water depth over their crowns.
You can also use a contour basket and gravel to raise a group of plants up if they need less water over their crowns. Add gravel to the contour basket first and then add the 3x1 litre mesh baskets to the contour and stand them on the gravel base layer. Fill the spaces around the smaller baskets so that the contour is filled with gravel.