Standard RG-58 will give you under half of a dB loss for that three foot run. While you can probably measure that amount of loss if you use the right equipment, your scanner won't even notice that. A strong signal will still be strong. A weak signal will still be weak. A very weak signal will most likely still be very weak.
A signal that is so weak that you can barely tell it's there through the noise, but not really make out what is being said may (just perhaps and only if it's really that weak) be weakened enough by that half a dB loss that it's no longer detectable. No real loss here since you couldn't make out the signal anyway.
Radio signals are like water in a bathtub. There's really just three states, not enough, enough, and too much (yes, you can have too much signal).
* Not enough is for really weak signals or the bath water level that's really too low to do anything with. The signal will be noisy at best and not really good to listen to comfortably, the bathtub will be slippery to step into but not good for bathing. This level is where you need to add water to the bathtub or for your scanner reception increase your signal or reduce your coax losses. For a good antenna installation, this condition is generally the exception, probably less than 2% of signal levels would be where adjustments would be required if the antenna and coax selection were good.
* Enough is fine. You can listen to the signal just fine, the water level is good for bathing. This state covers probably 90% of the signal levels (or bathtub water levels). You could add more signal (or bathwater) or even remove some and it won't make any real difference.
* Too much isn't good. For a scanner, you'll overload the receiver and cause issues like images (signals appear where they don't actually exist), the overly strong signals will prevent the weaker ones from being seen by the scanner, or the circuits in the scanner will distort the signal's waveform so it won't be decoded properly. For the bathtub, you'll overflow the tub and have water all over the bathroom floor.
Small changes in signal levels are really meaningless unless it moves you from one of the above states to another. As in the bathwater example, say you fill the tub with water so there's "enough" water in the tub. You get a glass of water, fill it up with water and dump it into the tub. You added water for sure, but can you really see the difference in how full the tub is? Most likely not. You could say the same thing if instead of adding water you removed a glass of tub water. Still no real difference in how full the tub is.
The real issue could be the definition of "small". If you're close to the area where the state changes (enough to too much for example) only a small change may be important (in the bathtub example a cup may be OK, but a gallon may be too much). If you're in the center of the "enough" range, several gallons either way may make no difference at all.
Getting back to the dB loss issue for your scanner, the same rules can apply. "That coax has 3 dB more loss than this one. That's half your signal!!!" OK, while this may be true, if the signal is on the high side of "enough", half your signal may push things down a bit to the lower side of "enough" and since "enough is enough", that loss is OK. (Think a tub 3/4 full of water and you drain half of it out. Still enough for a bath!). Now if that signal level was just a bit above the "enough" level, a much smaller loss level may be important.
Generally though, loss or gain values in the fractional levels (often as much as 1.5 dB) are not enough to change the signal state. As they often say, don't sweat the small stuff really applies here. If the loss is small, don't worry about it!
