@smoothrunnings, I'll try to answer all of the questions you've asked. Hopefully I don't miss any.
Latency for your miners is not an issue in your case. You are running a local node with both the miners and the node on the same physical network. Latency between your node and others is unknown to me. You can check it by pinging the nodes to which you are connected. The latency between your node and others is important because the faster you can propagate your found shares, the better the chance they aren't orphaned.
You can also help ensure you're getting BTC blocks relayed as quickly as possible by using Matt Corallo's relay node. Check it out here:
https://bitcointalksearch.org/topic/how-and-why-pools-and-all-miners-should-use-the-relay-network-766190.
Regarding your question about earnings, I've got only one word for you: variance. Sometimes you're going to be unlucky and not have very many shares on the chain. Other times you're going to be lucky and have more than you should. Also, you need to remember. You are comparing earnings per day to earnings per block. Online calculators report earnings per day. As of right now, your 3TH/s expects to earn 0.05058
BTC per day. Your node reports expected payout per miner per block at that particular time.
The simple fact is that p2pool is going to be affected by variance because of its relatively low hash rate compared to the rest of the network. In addition, you as an individual miner on p2pool are affected by variance because of the nature of finding shares. It's a double whammy that leads to significant swings in earnings. I'll give you a concrete example of this. On 9/10, p2pool found 6 blocks. I also happened to be on an upswing and had more shares than expected on the chain. I earned 0.26259819
BTC that day (an average of 0.043766365
BTC per block). Way more than what I expected to earn for my hash rate from my 8 S3s. On the other hand, p2pool found only a single block on 9/12. That day, I only earned 0.03200959
BTC.
Here's another example. I got a co-worker of mine into BTC and he currently owns an S3. He's currently mining on my node and hasn't found a share in over 5 days. He's missed out on 10 blocks so far. I hear the same thing from him constantly, "What's wrong with my miner? Why haven't I found a share? Should I reboot it? Something must be wrong! I'm going to switch to another pool." Then I show him that even with not receiving payments for some blocks, he's
still ahead of expected earnings since he's started mining on p2pool.
In short, it's all about the variance. If swings in luck are too much for you to deal with and you want the comfort of seeing lots of small, regular payments adding up, then p2pool is not for you. I'd suggest one of the more traditional pools. Otherwise, sit back, relax and enjoy the ride.