It's a three star shield with no alchemy path just like the Swiftstrike Buckler, except it also protects against elemental damage as well and supplies and a two health bonus instead of an attack speed increase. Would you ever use it for tier 3 levels?
If not, then how much of a health bonus would you say it would need to have to be a considerable shield to use in tier 3?
Unlike the Swifty, the Scarlet Shield has a mainly mediocre bonus. I would never use it in T3 (unlike the Swifty, but ima kinda good at dodging and it is kinda OP).
For it to be viable in T3... It'd need a +12 HP bonus IMO.
That is why it stops at 3*.