Can i wire 3 jbl gto1204d subs into 1 jl audio 500/1 amp

You can connect them in a series/parallel configuration. Put two of them in series and then connect the last one in parallel across the two in series. This would give you an impeadance of around 2.7 ohms. Only do this if your amp is stable at 3 ohms or less. I am not familiar with the specs of your amp. If your amp is only 4 ohm stable you can not do this.
This would also give you half the power on the two subs in series as the power for the one that is parallel. The two in series would be sharing the total output signal, where-as the one in parallel would get the entire output signal from the amp.
if you were to connect a fourth sub woofer in this configuration it could give you a 4 ohm load, by adding the fourth sub in series with the single sub that is in parallel. or in other words, yu have two sets of 2 sub woofers each in series. That gives you two 8 ohm loads (two 4 ohm subs in series is 8 ohms). Then you take the two sets of subs and parallel them (two 8 ohm loads in parallel equals 4 ohms).

If i had a picture to show you it would make sense, two subs in series that are in parallel with two subs in series.