Hi, well the answer is it doesn't matter as long as you have cabled yourself up correctly, i.e the DCE end of the cable is connected to the router you intend to use to issue the clock rate. I am assuming you are in a home lab situation and have purchased a couple of DB60 DTE DCE Crossover cables to directly connect your routers. Theses 'lab' cables are clearly marked which end is which - but if yours aren't then issue the 'show controllers' command and read carefully.
To start with please note the following key: ROUTER = R
I hope I understand what you mean. Considering the number of routers you have, I presume you want to connect all 3 together yes? Ok, if that was a yes, then I will say the following. I have 4 routers and three are connected together. The other 1 is just separate, and I just use it for command lines and play trials, so forget about this one for a minute. In addition to this, I have 2 DTE/DCE cable and 3 console cables [DB9 to RJ45]
I have connected my 3 x Routers as follows:
R1 - serial 0 [DTE] connected to Router2 - serial 0 [DCE]
R2 - serial 1 [DCE] connected to Router3 - serial 0 [DTE]
Please note that, in this connected method, I have considered Router2 as my imaginary ISP, and the fact that some routers may not have the ability set a clockrate. The rule taught to me is that, You must use the clockrate command on the DCE end of the connection in order to bring the line protocol up. I also remember learning that, when we are connecting a Router in an office environment we are supposed to connect the DCE end to the Internet Service Providers supplied unit or end. In this case we are lookign athe fact that we are setting up a home lab [or well thats what it is for me].
With all Routers connected, passwords enabled and interface IP addresses set on the serial ports and ethernet [or maybe you may have a fastethernet port] I have the following configurations set on Router2 having the DCE end of my DTE/DCE cables and all works well with the following results as per attached.
R2(config-if)clock rate 64000
R2(config-if)clock rate 64000
This is to the best of my knowledge and would advise you get other opinions also considering I am not yet CCNA and just about to have my exams in the next few weeks.
Kindly note that the attached note is results of my #show ip route [command], looking at the fact that I will be configuring static and other routing functions later in the week then wind up with frame relay configurations trials.
Thanks and I hope this is understandable and helps you.
The other piece of wisdom to remember for your CCNA exam, is DCE is used to send the clock rate. Just remember (in your own mind that) the 'C' in the middle of DCE stands for 'Clock' (it stands for 'Circuit' in reality, but just think 'clock' in the exam and you will put the clock rate on the correct end) and you can't go wrong. If you have never heard of Jeremy Cioara or Chris Bryant then you need to do some serious research and I urge you to buy their study materials from CBT Nuggets or Train Signal. I 'did well' on my CCNA. Truly money well spent I guarantee you.
Good luck in your Cisco certification pursuit, it's made a massive difference to my income.