As Whitelion says, don't. Not worth the time and effort for something that won't last. If you are not too confident with building a timber or block wall Bunnings have products to keep it simple,
https://www.bunnings.com.au/50-x-50-...right_p1080307 . You can use the companies concrete retaining boards or with these you can use treated pine, If you are only going up a total of 1.5m with 3 walls it will be simple to achieve and pretty cost effective. There is a link on the Bunnings page and it even explains everything you need to do. The link block walls where you don't need mortar are also quick and relatively cheap to do but you will need to be able to get the base course really level. If you don't want a professional in, base your choice on your skill level and confidence.