The following minimal working example shows how one can change bullet style of different itemize levels using \setbeamertemplate command:

% Bullet style in Beamer

% Theme choice

% set the itemize item symbol as a diamond

% set the itemize subitem symbol as a triangle
\setbeamertemplate{itemize subitem}{\scriptsize$\blacktriangleright$}

% set the itemize subsubitem symbol as a circle with a dot
\setbeamertemplate{itemize subsubitem}{\scriptsize$\odot$}


\frametitle{Bullet style in Beamer}
        \item Level 1
            \item Level 2
                \item Level 3


Compiling this code yields:

bullet style in Beamer

We used diamond shape for main items, right triangle for subitems and a circle with a dot for subsubitems.

For more details, I invite you to check Lists in Beamer – Complete Guide.