How to use alert(),confirm() and prompt() function using Blazor?

Unfortunately, there is not implementation of such useful functions in Blazor yet.
So you need to use JSRuntime instance.

@inject IJSRuntime JsRuntime

...

@code
{
    //...

    await JsRuntime.InvokeVoidAsync("alert", "Warning!"); // Alert

    bool confirmed = await JsRuntime.InvokeAsync<bool>("confirm", "Are you sure?"); // Confirm
    string prompted = await JsRuntime.InvokeAsync<string>("prompt", "Take some input:"); // Prompt

    //...
}

It makes possible to execute JS code right inside your C# code. With that you can use any JS logic you want to create behaviour you need.

See docs for details.