The main reason for the tripartite struggle between Palas, the Gurjara-Pratiharas and the Rashtrakutas was the desire to possess the city of Kanauj as it was the symbol of sovereignty over the Ganges plain.
For further reading check the following articles:
|100 Differences Between Articles||UPSC Prelims|
|NCERT Medieval Indian History Notes UPSC||UPSC Mains Questions (2013-2019)|
|NCERT Ancient Indian History Notes UPSC||NCERT Modern Indian History Notes UPSC|