Dropping stitch question

Hi guys, I’ve knitted a few basic baby cardigans, hats etc but I’m picking back up after years of no practise and am finding myself baffled with certain things. I’m currently doing good a Cardigan where at this point I should have 12 sts left on my needle, so far so good. But it’s now saying the following ‘work 3 rows dec 1st at armhole edge in next and every following alt row AT SAME TIME dec 1st at front edge in next and every following 0 row. Left with 9 sts’. Now because it says next row I have assumed that to mean I dec 1st at armhole edge in the second of the 3 rows I have to do and also dec 1st at front edge of that same second row. But that means I’ve only dropped 2sts and will therefore be left with 10sts and not 9 as the pattern says I should. I apologise for the long rambling message but any advice would be much appreciated x

Welcome to KH!
You’re up to working rows 1-3. The next row is row 1. Decrease at the armhole edge on rows 1 and 3. Decrease at the front edge on row 1. That’ll give you the 3 decreases that you need.

So happy somebody responded thank you so much. It now says I should continue until armhole measures 8cm. Where should I start measuring from exactly to get that 8cm? Thank you x

Is there a cast off at the beginning of the armhole? If so, measure straight up from the cast off row.
If the armhole just begins with a decrease, then measure straight up from that decrease row.
What is the name of your pattern?