Can I claim a business computer purchased a few years ago as an asset?
If you use your computer, tablet and software in your business, then yes, business assets can be depreciated. You would need TurboTax Self Employed (online) or TurboTax Home & Business (desktop) for a Schedule C business. Enter under the Asset section of your business.
The IRS will assume that you have taken the depreciation expense in prior years whether or not you did. You would be claiming the current year and then going forward any remaining.
You could amend past returns and take the depreciation that you should have (when the IRS makes the form available). Amend instructions are below.