One thing you will find on this forum (and others I suspect too) is that people generally won't straight out give you the answer to the problem you pose. This is great because it really forces you to learn about the ins and outs of the project you are making.

A good way(best way?) to design a product is to start with a set of end goals (which you have

) and slowly expand on how you will need to get to the required solution.
For Example, your goal is to have a dual tracking ±0-30V 0-3A adjustable Lab power supply with current limiting. Your next question would be output regulation- how much ripple, etc. do you want on the output? If you want a low noise supply, you should consider linear. If you can tolerate some ripple, switching might be a good option. If you want improved efficiency, maybe you could consider a switch-mode pre-regulator feeding into a linear regulator.
Do you need to use a Linear regulator? Can you use a power transistor?
Then consider where you are getting the power to be able to feed 180W through both outputs. are you going to be using some form of battery, or rectified AC from a transformer? if you go the transformer route, will you need one transformer (centre-tapped) or one for each output? make sure you consider that you will also need to use power in the control circuitry, so your transformer will need to be able to provide over 180W.
You mentioned Current limiting. That means you will need to measure the current flowing, in both the +ive and -ive outputs. How are you going to do that?
How are you going to control the output voltage and current limit? are you going to use potentiometers, or do you want to use rotary encoders, and MCU and DACs to drive the outputs?
How are you going to display

the output voltage/current? are you going to read them to an MCU, or use panel meters?
At this point you can start to consider physical issues. First, identify what parts will be hot. These parts will need to placed on sinks that will be large and bulky. Consider your transformer. It will be the largest component you have to house. You will also need to be able to place all the displays, output control circuitry, and output connections on the front, and you will need it all to fit. You will also need to consider that your control circuitry might be bulky, so you will need to leave space for that.
Now we come to PCB layout. First, you need to find what limits your board will need to adhere to. Things such as the maximum allowable board size, and importantly how the board will be mounted. Will the board be in rails, or will it be mounted on standoffs/screwed down. Either of these is going to mean that there are regions on the boards that will be off limits, which you will need to account for.
Before you consider component placement, take a moment to think track size. Some tracks-especially any carrying high currents-will require thicker tracks. There is also the trend to go to the smallest track size possible too. Do your tracks need to be at the bleeding edge of circuit design, or could you possibly get away with 16mil tracks?
Now we can start placing components, using a logical manner, I.E. keep parts of a circuit within the same function together. Again, keep an eye out for clearance or heat issues. And don't forget those decoupling caps!
Once you have laid out the board, go back and review all you have done so far. If anything needs to be tweaked or changed, do it.

So this is just a start: Every little thing you need to decide will have a number of flow-on considerations that you will have to take into account. Its also ALL OF THIS that all of us electronics enthusiasts love about our hobby/job. You will also find that for every question, you will find 5+ solutions. So take heart young Padawan! You have a long way to go yet!

-kizzap
(as an aside, make sure you go watch a lot of the earlier videos our humble leader has uploaded to youtube. There is a wealth of information there that will help you on your journey.)