Depending on the headroom required by the regulators, and the voltage drop across the bridge rectifier diodes, a 24V RMS secondary could give up to 20V regulated DC out at a DC current of 60% of the secondary's RMS current rating. Depending on the transformer regulation, the unloaded unregulated voltage could be as high as 37V, possibly a bit higher for a really crappy (poor regulation) transformer, and if the mains supply voltage is significantly higher than nominal it could rise to 41V.
The center tap (C.T.) of a winding feeding a bridge rectifier is at half the unregulated DC output voltage from that bridge rectifier, so if it is grounded, it splits the output into equal positive and negative rails, though the reservoir caps should also be split so they are between Bridge+ and C.T, and C.T. and Bridge- respectively.
A bench supply with dual outputs, positive and negative is more useful than a single output supply of double the voltage, and if you want the full voltage you simply wire the load across the +out and -out. Getting the peak unloaded unregulated voltage under 40V makes the regulator design many times easier.
Therefore I'd start off with making a dual output +/- 1.2V-18V supply using a LM317 for the positive rail and a LM337 for the negative rail. Given good heatsinking it should be able to supply up to 1A. If the transformer's unloaded rectified and smoothed peak DC output voltage is above +/- 36V (with respect to the C.T.), it will need preregulators to limit the voltage to the adjustable regulators.
Then start working on a design for a +/- 0-15V supply with adjustable current limiting, say 0-500mA, with tracking positive and negative outputs, that you can test running off the +/-20V supply you've built, and when you've got that working, figure out how to boost it to +/- 0-20V, 0-3A, replacing the LM317/LM337 regulators to use the full rating of the transformer. Warning: this will be *TOUGH* even after you've gained a lot of experience with the LM317/LM337 supply.