BlazorFiddle - Blazor .Net Developer Playground and Code Editor in the Browser.

Index

@page "/"

Check 3 Items:
<ol>
    <li><input type="checkbox"  @bind=@Checked[0]  disabled=@(!Checked[0] && IsLocked ) /></li>
    <li><input type="checkbox"  @bind=@Checked[1]  disabled=@(!Checked[1] && IsLocked ) /></li>
    <li><input type="checkbox"  @bind=@Checked[2]  disabled=@(!Checked[2] && IsLocked ) /></li>
    <li><input type="checkbox"  @bind=@Checked[3]  disabled=@(!Checked[3] && IsLocked ) /></li>
    <li><input type="checkbox"  @bind=@Checked[4]  disabled=@(!Checked[4] && IsLocked ) /></li>
</ol>

IsLocked: @IsLocked

@code {

    bool IsLocked => Checked.Where(p => p).Count() >= 3;

    bool[] Checked = new bool[10];
}

namespace BlazorFiddleProject
{
    using Microsoft.AspNetCore.Components.Builder;
    using Microsoft.Extensions.DependencyInjection;


    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
        }

        public void Configure(IComponentsApplicationBuilder app)
        {
            app.AddComponent<App>("app");
        }
    }
}

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8"/>
    <meta name="viewport" content="width=device-width">
</head>
<body>
    <app>Loading...</app>
    <script src="_framework/blazor.webassembly.js"></script>
</body>
</html>

BlazorFiddle was updated from Blazor 0.7 to .NET 6.0. Your old source code could not work. You need to upgrade to latest.