The tensile strength of concrete is never very high, even when fully cured, so at 3 days, don't count on anything.
Also, tensile strength is more variable than compressive strength. While there is a relationship between the two, in my experience I've seen the tensile strength be more variable.
I'm curious as to why this is important at 3 days. Are you concerned about direct tension or modulus of rupture for flexure?
You can also do a splitting tensile test on cylinders molded for compressive strength.
Assuming the tensile strength at f'c is 10% of f'c, then the tensile strength at 3 days would be about 0.4 to 0.5 times that value, assuming approximately 22C ambient and a normal mix (w/c about 0.50 or less) without flyash or GGBFS. So...
For 30 MPa mix, ultimate tensile would be about 3 MPa and at 3 days, about 1.5 MPa.
These are rough correlations that can be found in most of the literature (for example, A.M. Neville...any edition)
both tensile and compressive strength is highly dependant on the mix design and curing conditions. for example if using high early strength mix design, it will be higher than if using a mix high in fly ash. Curing temperature will also play a significant role in strength gain. the best source for this is test results from the actual mix. ask your ready mix supplier for it.
