In the name of Allah the Most Gracious the Most Merciful
The answer to your question is as follows:
According to Shariah ruling, you are not Mazoor. Hence if the drop of urine comes after urination, it will surely nullify your wuzu. You must change napkin, make Wuzu and perform namaz. Changing napkin twice or thrice is not sufficient. Every time the napkin gets wet you have to change it, renew your wuzu and read namaz.
NOTE: A Mazoor is one who suffers from urine problem and cannot find so much time that he can perform wuzu and one namaz without urine drop. If full time of a salah passed and he could not be able to perform namaz with complete purity, then he is Mazoor. And he will be Mazoor as long as full time of a namaz passes without urine problem.
For him, Shariah gives concession that he must perform wuzu when the time of a salah starts and then perform namaz even with the drops of urine coming out and falling onto his clothes. He can complete his salaah in that state.
For more, See Fatwa No. 971 online
And Allah knows the best
Darul Uloom Waqf Deoband