For Gaura-Nitai alone, Rathayatra is not at all recommended because They are playing the part of devotees. But as you are having three large carts you may arrange the following: Sri Sri Gaura-Nitai in the first ratha, Sri Sri Radha-Krsna in the second ratha and Sri Jagannatha, Balarama and Subhadra in the third ratha.
Letter to Ramesvara, 17 April, 1977