وَمَا شَرُّ الثَّلَاثَةِ أُمَّ عَمْرٍو بِصَاحِبِكِ الَّذِي لَا تَصْبَحِينَا
O Umm ʿAmr, the one you do not serve in the morning is not the least worthy among the three.
Mā … biṣāḥibiki is an emphatic negation with an additional bāʾ in the predicate. Sharr al-thalāthah means the least worthy of three companions.
The complaint therefore concerns standing and fairness in the group, not simply thirst. The poet’s claim belongs to the particular pre-Islamic gathering he describes.