Solo(k) - And whether/when 5500's are required

A business has 3 unrelated owners who each own 33 1/3% of the business.  The business has no other employees.  Can this business sponsor a single plan that is considered an owner only/solo plan which is exempt from nondiscrimination tests and 5500 filing?  The business is structured as an LLC and taxed as an S-Corp.  

From the 5500-EZ instructions

Who Must File Form 5500-EZ You must file Form 5500-EZ for a retirement plan if the plan is a one-participant plan or a foreign plan that is required to file an annual return and you do not file the annual return electronically on Form 5500-SF. A one-participant plan means a retirement plan (that is, a defined benefit pension plan or a defined contribution profit-sharing or money purchase pension plan), other than an Employee Stock Ownership Plan (ESOP), which: 1. Covers only you (or you and your spouse) and you (or you and your spouse) own the entire business (which may be incorporated or unincorporated), or 2. Covers only one or more partners (or partners and their spouses) in a business partnership, and 3. Does not provide benefits for anyone except you (or you and your spouse) or one or more partners (or partners and their spouses). A one-participant plan must file an annual return unless the plan meets the conditions for not filing under Who Does Not Have To File Form 5500-EZ below. A foreign plan means a pension plan that is maintained outside the United States primarily for nonresident aliens. A foreign plan is required to file an annual return if the employer who maintains the plan is: • A domestic employer, or • A foreign employer with income derived from sources within the United States (including foreign subsidiaries of domestic employers) if contributions to the plan are deducted on its U.S. income tax return.

 A single plan can be established for them, and as there are no NHCEs participating, nondiscrim tests are n/a.  If the business entity can be construed as a partnership, a filing would not be needed until plan assets exceed $250,000, otherwise a 5500-EZ would be required when such threshold is reached.  If it can't be considered a legal partnership, a 5500 or 5500-SF would be needed right off the bat, generally depending on what type of assets are held in the plan.   

Thanks for your replies.  So, if you have an LLC being treated as an S-Corp and there are 3 separate owners (with no spouses or common-law employees) then you have to file the 5500-SF, right? 

Would they be eligible for the $500 tax credit? 

The tax credit is tied to how many non-HCEs are covered, since this plan covers no NHCEs it is not eligible for the tax credit.

Thanks everyone for the quick replies. Hope everyone has a happy holiday.

I'd note that the IRS is now (more clearly) stating that 2% S-corporation shareholders are considered "partners in a partnership" for 5500EZ purposes: https://www.irs.gov/retirement-plans/file-your-one-participant-plans-electronically-using-form-5500-ez

I wish we had that clarity on if they are considered such under 29 CFR 2510-3.3 for purposes of being exempt from Title I of ERISA. If such clarity exists for ERISA-exemption, let me know!