Responsive Notification page using React components

Anyone have a good explanation on how I can target my .class:nth-child()? Say I have 7 mapped elements with the class named .title. How do I target only number 3 and 7 in CSS? Element:nth-of-type() works well and its straight forward, but I found targeting class:nth-child() to be way more difficult than expected. I read the documentation and I understand that I need to use a formula, but that's about where my understanding ends. I just gave up and I ended up using a work around, which I'm not overly thrilled with.