With an NPN pass device your op-amp needs to drive the base higher than your expected output voltage,
**broken link removed**
For single supply operation the LM741 is only rated to a 22V rail which, if you are supplying it from your 13.6RMS/"18V rectified" supply would be OK.
However if you look at the available output voltage swing, and possibly current drive,
Then it suggests that the op-amp might only be able to raise its output, under load, to 5V below the supply input. So 18V-5V is 13V and then you have to account for the Vbe drop of 0.7V for your pass device and the maximum, nominal, output becomes 12.3V which sort of fits in with what you are seeing.
Regarding the maximum available current from your LM741 25mA typical is indicated. For a 2N3055 the current gain, hfe,
http://www.energies.alba-annuaire.fr/data/tr/2N3055.pdf
is specified as 20-70. You can also see from the above that the Vbe exceeds the 'rule of thumb' 0.7V at 4A causing more problems..
With 25mA typical available from the LM741 then using the maximum hfe of 70 your circuit would only be able to supply 1.75A, Ib.hfe, before it gave up.
There are 3 amp adjustable regulators available...
3A 3 terminal regulator - Google Search
http://www.fairchildsemi.com/ds/LM/LM350.pdf
However that one has an NPN Darlington configured output,
and as a result suffers from the same voltage drop out problems,
So you might consider a LDO, Low Drop Out Regulator..
3A 3 terminal LDO regulator - Google Search
Linear Technology - LT1085 - 7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators
http://cds.linear.com/docs/Datasheet/108345fg.pdf
Grrrrrrrrr, which in itself is still using NPN pass devices and still needs 1V of headroom which will not be, or rather just is, available if you want 17V out from your 18V nominal input supply. Sorry, I'm just doing searches and hitting links, or lack of them. There will be more suitable devices out there. I'm not trying hard enough to find one but they do exist.
Perhaps you are ending up having to use such a large filter capacitance on your supply in order to avoid ripple affecting the regulator and probably need more input voltage. However, if I have read correctly, 18V in with 17V out is probably going to be unworkable so it looks like you will need a different transformer.
Genome.