Please, please, please tell us that you aren't actually considering not reporting the wages from your third job on your tax returns because it results in a smaller tax refund. The W2s that you received from your employers are multipart forms marked Copy B, Copy C, Copy 1, etc, and guess what, Copy A, which you never get to see is not saved in the employer's files as a record of what was paid to you. No, Copy A is sent to the IRS and so it already knows that you received the wages reported thereon (and also how much tax was withheld). If you do choose not to report some wages on Form 1040 at all, your return will likely be "accepted" because the initial checking is simply an arithmetic check; did you make any numerical errors in filling out things like "Enter the sum of Lines 12, 13, and 14 here" based on what you wrote on Lines 12, 13, and 14? A refund check might also be issued. But, later in the year, the IRS will be checking your return against all those Copies A of your W2 forms that it has (ditto Forms 1099-INT and 1099-DIV etc) and will discover that you didn't include one of your W2 forms at all. At this point, you will get a letter from the IRS about the matter, demanding a return of part of the refund that you claimed (because you claimed a larger refund than you were entitled to) and possibly assessing interest and/or penalties. TL;DNR version: don't do it.
Actually, what you want to do is not to try to find ways to maximize your tax refund but instead try to find ways to minimize the tax that you pay (the "bottom line" of Form 1040). A tax refund merely means that too much money was withheld, and if you would like to get a larger refund for 2019 than you got for 2017 (or will be getting for 2018), there is a very simple stratagem. Just file a new Form W4 with your employer asking that more money (say $100) be withheld from your pay each month and sent to the IRS as additional income tax, and voila', you will get a larger refund in 2020 after you file your 2019 return. You will have, in effect, made an interest-free loan to the US Government (and the nation thanks you for your patriotic spirit) for the ineffable pleasure of receiving a large tax refund. What you ought to be doing is thinking of ways of reducing what is due from you for 2018 taxes (and yes, this too will increase your tax refund for 2018). As the answers from Justin Cave and JoeTaxpayer say, about the only option is to fund a Traditional IRA for 2018 which is not appealing to you at all as per your comments on those answers. So, do nothing with regard to IRAs, fill out your Form 1040 accurately, including all the W2s for 2018, and accept the smaller tax refund for 2018. Whether you wish to cuss out the Tax Jobs and Cuts Act of 2017 which took effect in 2018 and which is causing all these problems for you while doing so is up to you.