3x35 or 3+35 simply means that you pay three payments in advance, followed by 35 monthly payments. You may see 6x35 advertised, which is fine except that it is sometimes used by companies to make the cost appear less. If comparing 3x35 with by 6x35 you need to do the calculation to establish which is the least expensive.
For example if the same vehicle is advertised, one with a profile of 3 x35 at £200 per month and one with a profile of 6 x 35 at £186 per month, the calculation is as follows: 6 + 35 totals 41 payments, 41 x £186 = £7,626 over the term. 3 x 35 totals 38 payments, 38 x 200 = £7,600 over the term. So whilst £186 per month appears more attractive than £200 per month, it is in fact more expensive