I assume that you have not applied to be treated as an S corporation and instead your LLC is treated as a disregarded entity and you will be reporting your business on Schedule C.
The elective deferral limit (for 2018, $18,500 plus, if over age 50, $6,000 catchup) is a per-individual limit. Since you've already deferred about $5k to the first employer, you're maximum elective deferral to the solo 401(k) will be about $13,500 (or $19,500 if you are age 50 or over in 2018). However, depending on your net profit from self-employment, you could be limited to less. Your total contributions cannot exceed your net profit minus the deductible portion of self-employment taxes.
Employer contributions are governed by a separate per-plan limit, so the $3k of matching contribution made by your first employer has no effect on how much you can contribute to your solo 401(k). If you have sufficient profit from self-employment, you'll be able to make an employer contribution to your solo 401(k). The calculation is a bit complicated since it's governed by several limits. The calculation is described in Chapter 5 of IRS Pub 560:
https://www.irs.gov/pub/irs-pdf/p560.pdf
You won't be able to use TurboTax's Maximize function for an individual 401(k) to do this calculation for you because this function in TurboTax doesn't take into account your elective deferral at your first employer. Instead, you'll have to manually enter the elective deferral that you are eligible to make without exceeding the individual elective deferral limit, then use the Maximize function for a SEP contribution to allow TurboTax to calculate just the maximum employer contribution. The maximum employer contribution for a SEP plan is the same as for a 401(k) plan.
