D&A in 3-statement model

First of all, let's start on forecasting depreciation. Overall, there's various methodologies when considering how to forecast depreciation in a three statement model (from what I've seen).Typically, in a simple model, you can either forecast depreciation by allocating the overall expense to a percentage of sales, a percentage of CapEx or setting it equal to CapEx. 

As for amortization, this is typically done in a similar manner to depreciation in a simplified model. Although there could be some wrinkles here in advanced models pertaining to the amortization of intangibles arising from purchase accounting.

Moving into how this ties into the modeling, it really depends on the model, especially considering how companies show their depreciation and amortization expense on the income statement versus the statement of cash flows. Specifically, one thing we have to keep in mind for the income statement is the fact that some depreciation and amortization is housed within the operating expense line items - you should be able to find the precise figures relating to operating expenses (SGA, R&D, etc.) allocation to D&A expense as a note to the financial statements in their 10-Ks. As a result, the statement of cash flows will provide more comprehensive picture of these non-cash expenses. That being the case, when we're tackling this problem in the modeling, we have to be cognizant to either, 1.) remove the entrenched D&A from the income statement items in our forecast and the historical period for an apples to apples comparison, or 2.) assume the D&A expense will remain in those operating expense line items and not show the expense as a separate line item on the income statement, but as a memo to the income statement.

Consequently, in your modeling, the total D&A expense will be the exact same on the income statement and the statement of cash flows.

As for the EBITDA question, again, this really depends on the modeler, as well as the company. I've seen models that don't show EBITDA as a line item, but as a memo and I've seen models that show the progression from gross profit to EBITDA to EBIT and so on. So, the approach you choose to calculate D&A on the income statement will ultimately decide how you want to approach the line items on the income statement. By way of example, if you want to show D&A as a separate item on the income statement, you can subtract the operating expenses from gross profit to reach EBITDA, then subtract D&A and stock based compensation (SBC) to reach EBIT, and so on.

EBITDA is also known as a proxy for cash flow, due to the fact the non-cash expenses are stripped from the calculation. This can overstate the company's performance in a sense, depending on who you ask - Warren Buffet is on record saying he prefers EBIT due to the fact the the 'toothfairy' doesn't pay for capital expenditures (same logic can be applied to SBC). With that in mind, if you're of the mindset cash is king, EBITDA is without a doubt one of the more prevalent methodologies in pricing deals.