It depends on whether the reimbursement will be taxable or tax-free.

On my w2 it is not taxable.

You will need to enter your employer's reimbursment in TurboTax under Education Summary, then select Scholarships and Grants. That will adjust your credit.  Enter your 1098-T information exactly as it is.

Because the reimbursement is not until 2018, you could claim $3395 as the tuition you paid in 2017.  However, that will make the reimbursement taxable in 2018.  The simpler thing to do is reduce the tuition you paid by the reimbursement you expect and claim the remainder for credit in 2017.  That way, no future adjustments or corrections will be needed.  

But my employer will show the tuition reimbursement on 2018 W2. At that time what should I do?

You said the reimbursement will not be taxable, that means it won't be on your W-2, or will only be in box 14 which is for information only and has no affect on your tax return.

Your employer does not have to include the tuition reimbursement in your pay stub. This is as long as you are not a highly compensated employee (meaning a 5% owner or earned $120,000 in the previous year). See IRS guidance: https://www.irs.gov/pub/irs-pdf/p15b.pdf

You will need to enter the employer reimbursement in TurboTax under the Education Summary section, select  Scholarship and Grant, Employer Assistance is on the next page.  This will adjust your education credit.

It will not affect your 1098-T.  That comes from your school and does not take into account employer educational assistance.

Your employer can use the tuition reimbursement as a business deduction up to $5,250.  Over that amount it must be included in your wages.

[Edited 01/17/18 | 6:18 pm]