1. Explain it simply
左右 means 'about/around'. Put it AFTER the whole amount: 三十岁左右, 八点左右.
几 means 'a few / -teen'. Put it before the measure word: 十几个 (ten-something), 几个 (a few).
多 means 'and a bit more'. For round tens it goes before the measure: 二十多块, 三十多岁.
For a single unit, 多 goes after the measure: 一个多小时.
2. Show examples
他三十岁左右。
Tā sānshí suì zuǒyòu.
He is around thirty.
左右 after the amount
有十几个学生。
Yǒu shí jǐ gè xuésheng.
There are ten-something students.
十几 = ten-plus
这本书二十多块。
Zhè běn shū èrshí duō kuài.
This book costs twenty-odd kuai.
多 before the measure
我等了一个多小时。
Wǒ děng le yī gè duō xiǎoshí.
Sandhi:Wǒ děng le yí gè duō xiǎoshí.
I waited over an hour.
多 after the measure (single unit)
3. Share a tip
左右 goes at the end of the amount; 几 goes before the measure; 多 goes before the measure for round tens (二十多块) but after it for one unit (一个多小时).