How can I pass variable value in Qweb when calling a function?

I have this code :

<t t-set="fname" t-value="o.firstname"/>

<t t-set="lname" t-value="o.lastname"/>

<t t-set="city" t-value="o.city"/>

<t t-set="getadd" t-value="o.get_address('Graham','Sarah','Paris')"/>

<span t-esc="getadd"/>

How can I pass the variables in the first three lines to the function call in line 4? I can't put a t-esc within a t-set .