You misunderstand 分 here.
分 does not mean "division" here. Here 分 means the "points" (test score), in an exam or a contest where the perfect score is 10.
分 can also means "perdec" (per ten, a tenth).
三分 3 points (out of 10), 3 perdec (3/10)
五分 5 points (out of 10), 5 perdec (5/10)
十分 ①[original meaning] 10 points (out of 10), perfect score, full marks ②[extended meaning] "perfectly", very, extremely
幾分 how many points (out of 10), how many perdec
Assuming that there is an exam called "love", how many points can I get from the exam?
我愛你有幾分?
How many points do I love you?
How many perdec do I love you?
= How much do I love you?
Extra examples:
三分天注定,七分靠打拼。 [famous lyrics]
Three points depend on fate, while seven points depend on hardworking.
3/10 depends on fate, 7/10 depends on hardworking.
[Chinese version of "nature versus nurture"]
逢人只說三分話,未可全拋一片心。[idiom]
Every time you meet a person, you should only say 3 perdec (3/10) of your mind. You should not "throw the entire heart to them" (tell all you mind).
As for proportion, Western culture prefers to use "percent" (per hundred, a hundredth), while Chinese culture traditionally uses "perdec" (per ten, a tenth), known as 分, 成, 折.
Examples:
三成 three perdec (three tenths, 3/10)
七成 seven perdec (seven tenths, 7/10)
九折 nine perdec remaining (i.e. 10% discount)
八折 eight perdec remaining (i.e. 20% discount)