Adding dependant sources in LtSpice (CDCS)
You are using the current element (as seen in with the F2 browser), which is a regular current source, as opposed to a behavioural source, which, for current, is bi and bi2. Once you place that, the syntax changes and you can write I=I(...), as the default placeholder suggests.

However, as per a recent answer, the manual warns you about quasi-instantaneous current feedback, and you should use an F source (which is, as you guessed, displayed as f with F2 browser). Once you place that, its value should be changed, just as the manual says -- and for your case, in particular -- to V1 3. Note that the current into V1 goes through the + pin, and exits from the - pin, irrespective of placement, so you should take that into account with a proper sign: V1 -3 (if that's the case). See the linked answer for an exemplifying picture.