多 is based on the words you wanted to talk. You can replace "x" (unknown) with it.
You don't always use measure word to depend where to put it in, it is based on how much is it. Normally, it filled up the empty space you were talking about.
- 三块多 = 3.x (local currency here you are talking about)
- 十五点八多公斤 = 15.8x kg
- 一点二五多公里 = 1.25x km
for larger number:
- 一千多单位 = 1xxx unit
- 十多亿美币 = 1,x00,000,000 (亿 = hundred million)
- 一万两千多点五八多公里 = 12xxx.58x km
- 十多点多度 = 1x.x degree
if you don't know how much is the number but how large is it (thousand, hundred, million etc), you can use 几 as an unknown (same use as 多, but much wider usage, normally used for asking)
- 那至少要几万元! = that cost at least x0000 (local currency here)! (normally more than 10000)