The three given reasons are intertwined, but if I must rank them, the priority would be “to know/love God”, “to grow in Christ”, “to serve God”. This is because the ultimate reason for growing and serving should be loving God. Otherwise your efforts may become just a duty or it is for your own glory. Moreover, knowing God is a primary basis. A person who serves God without God is quite dangerous: he may fall into and lead others into error. Secondly, growing in Christ should come as a higher priority to serving God. We can only produce and reproduce what we already are/have. So we should be overly preoccupied with serving, if we aren’t growing at the same time.
Finally, I wonder whether defending the truth against error should be a fourth main reason for Bible study. This, after all, was the context why Paul told Timothy to be able to handle the word of God correctly in 2 Tim 2:15. There were false teachers who were threatening to mislead the church.