Can i use IMediator in handler ?

[Pobx]

Hi, everyone. now i confuse about how to use mediator. normally, i use one to one(1 command/query with 1 handler). but last few days ago. i have a quest about how use interface IMediator in handler can we use or not ? for example as below

we have 3 query as below
queryA
queryB
queryC

1. we use all query in side new hanlder
   queryAllHandler

var a = _mediator.Send(queryA);
var b = _mediator.Send(queryB);
var b = _mediator.Send(queryC);

return new All{ a, b, c }

2. we use all query in controller
   method: queryAll in controller

var a = _mediator.Send(queryA);
var b = _mediator.Send(queryB);
var b = _mediator.Send(queryC);

return new All{ a, b, c }

which one we choose use ?