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Since TYPO3 8LTS

Since version 8 TYPO3 uses the standalone version of fluid, that was heavily developed and got tons of new features like elseif:

<f:if condition="{var} == 'something'">
    <f:then>do something</f:then>
    <f:else if="{other-var} == 'something else'">do something else</f:else>
    <f:else>do the other thing</f:else>
</f:if>
In addition there is support for syntax like this:

<f:if condition="{something} || {someOtherThing}">
    Something or someOtherThing
</f:if>
Until and including TYPO3 7LTS

With Plain Fluid you can nest two if ViewHelper:

<f:if condition="{var} == 'something'">
    <f:then>
       // do something
    </f:then>
    <f:else>
        <f:if condition="{other-var} == 'something else'">
            <f:then>
                // do something else
            </f:then>
           <f:else>
               // do then the other thing
           </f:else>
        </f:if>
    </f:else>
</f:if>
Or you could implement your own ViewHelper or use a ViewHelper Library like VHS that have a ViewHelper that does this more elegant.

https://stackoverflow.com/questions/29461381/fluid-if-then-elseif-else
https://docs.typo3.org/typo3cms/ExtbaseGuide/Fluid/ViewHelper/If.html