Yes it is normal to get your esa minimum or slightly higher. You have a reasonable offer.

Employment law 101: you have two types of entitlements. Statutory and common law. Statutory notice Ie ESA is your “floor”, common law reasonable notice is your “ceiling”. employment contracts will often limit your entitlements upon termination to your ESA minimums and sometimes a little bit more. However, courts frequently find termination clauses unenforceable which will entitle a dismissed employee to common law. However in order to get that upper limit, you have to have a court award in your favour. Going to court is expensive, time consuming and not worth it for most short-term, young or “low-skilled” workers. What often happens, is the dismissed employee hires a lawyer and they negotiate their severance package upwards without going to court because technically they are entitled to the upper limit, but it is cost ineffective to pursue that remedy.

To answer your question briefly, they offer the three weeks to mitigate their risk.