Help reading pattern- armhole shape

Please help I think I’m reading this wrong.

Dec 1 st at each end of next 3. Then on 1 fol alt rows. 27 sts.
Cont without shaping until armhole measures 20cm

So i’ve Done the first three sts decreasing at each end then I did 1 decreased and should I continue but dec on alt rows through the next 27? Bit panicked as it would only leave me with 7 stitches at the end which doesn’t look like the back of a jumper!

Please help x

You’ve decreased at each end of rows 1,2, and 3 so that’s 6sts decreased. Now dec on row 5 which is the one following alternate row. That’s 8sts total decreased. You should end with 27sts on the needle.

Thank you so much! I really appreciate your reply and I totally understand it now, thanks to you!

First time jumper making and I want to get it perfect!