Farmer Bills are constitutional in view of the Entry 33 of Concurrent list which empowers Central Government to pass Laws relating to Production, trade Commerce and Distribution of Agriculture products. States are having powers to make laws as empowered by Entry 26 & 27 but it is subject to Entry 33.
All the bills are passed by Lok Sabha and Rajya Sabha and even if it will be presented in Joint Sitting of LS and RS. present NDA government is in majority with 353 members in Lok Sabha and 112 in Rajya Sabha out of total members i.e 543+242=785
In my view it could not be challenged under Article 246 & 254 but could be challenged under Article 13. The views are personal and should not be relied upon without obtaining Legal Advice from Expert Lawyers.