I am not an accountant but this is my understanding on IRAs...
Not all traditional IRAs are deductible. If you are an active participant in an employer sponsored plan you can't deduct if your AGI is over $60,000 on a joint return. Also if you have AGI over $160,000 then you can't do a deductible contribution either.
If you qualify for ROTH it is better than a non-deductible IRA. You can't contribute to a ROTH if your AGI is over $160,000 for married people with a joint return (it is phased out from 150,000 to 160,000)
There are also spousal IRAs and you can rollover a regular IRA to a ROTH. If you do a rollover then you must pay taxes on the amount. Rule of thumb is if you are over 50 it probably isn't worthwhile, except for maybe estate planning purposes.
The $3,500 limit is for those over 50, otherwise it is limited to 3,000.
Also, to contribute to any of these you need earned income. Capital gains don't count.