Lucknow University is the best among these for B.Com. The university has a great commerce faculty. It offers great academics. It also organizes various workshops and seminars around the academic year. Around 70-80% of the students get placed in the B.Com course. The highest CTC offered is around INR 9 LPA. Students are usually recruited as accountants or managers.
Both Allahabad University and Lucknow University are tremendous universities. The facilities and environment are more or less the same in these universities. The academics are similar to an extent as well. The benefit of being a part of Allahabad University is that it is known for its tag of the central university and gives it an upper hand over Lucknow University which is a state university.
Your admission to both these universities depends on your UGAT results. Hopefully, you have secured a good score to be able to get into them. If this is the case, you should opt for Allahabad University. Allahabad University offers B. Com Plan whereas Lucknow University offers B. Com Honors.
Moreover, Allahabad University conducts annual exams while Lucknow University is known to conduct semester exams. If your goal is to concentrate only on B. Com then Lucknow would be the best pick. But if you want to prepare for additional courses such as CA, CS, SSC bank, etc., as per your choice then Allahabad University would provide you with enough time to prepare for it.