"得(dei)"in this sentence means "have to "or "need ","must " .it is normal that you don't know how to use "得"and "的" .sometimes even chinese make the same mistake . it is hard to explain the difference between them ,but i will try "得"has three pronunciations, "dé ","děi"and "de " 1、when "得"read as "děi",it means "have to ","need " ,"must " , the basic structure is "得"+v for example ,我得走了，我每天得吃三次药； 2、when "得"read as "dé",in most cases ,we take it as verb ,adj,interjection for example .我终于得到（verb）你了。看着他洋洋自得（adj）的样子，我心里很不高兴。得（interjection），就这样吧。 3、when "得"read as "de " it is used as auxiliary word ,the basic structure is "verb +得"（他去得，为什么我去不得？），"verb +得+complement"（听得懂/他回得来吗？）"verb/adj +得"（玩得不开心/吃得不好） "的"also has three pronunciations"de ","dì","dí" 1,when "的"read as "dí ",it is used as adj (实实在在的） 2、when "的"read as "dì ",it is used as n (标的） 3、when "的"read as "de",it used as auxiliary word ,the structure is 1,聪明的孩子（clever child)/我的父亲(my father)2，我在上海买的包(It was in shanghai that I bought the bag)3，他明天会来的(surely he'll come tomorrow)/屋子里静悄悄的(It was very quiet in the room)4、我喜欢这部电影的女主角(i like the leading lady of the moive) i hope i can help you

得 has 2 pronunciations. One is DEI, when it pronounces DEI, it means HAVE TO. The other one is DE, same pronunciation as 的。 Most of the time V+得+complement, the complement is emphasizing the verb. sth/sb +的 + noun, it expresses belong-relationship.

in your case,得 =必须 or 必须要=have to ，which means "i have to take three pills a day".“得” is used more in oral speaking ,daily use,it is not formal。 "的" will never be used in this way. I give you another example:oh ,it's so late ,i have to go!in chinese it is :很晚了，我得走了（or 我必须要走了） and pay attention to its pronunciation，in this case ,it is děi.

得 = 要； like 我每天要吃三次药 的：用在这里没义意

i need to take madicine three times a day. 得 is equal to "need to", and it is phoneticized "dei" in chinese, not "de", so 的 can't be used here.