Hi. The usual standard line level voltage in the U.S. is +4 dBu, but that is only a nominal level ("0 VU"); peak levels are considerably higher, e.g. +20 dBu or even slightly more, depending on the particular equipment and the signals driving it. So if your recorder can only take -3 dBV before overload, you'll need more attenuation than you seem to expect. To reduce +22 dBu (= 9.76 V) to -3 dBV (= 708 mV) would require 22.8 dB attenuation, for example.
I don't know the recorder, but if it has that sensitive a line-level input then I assume we are talking about a consumer-type input which is unbalanced, so you will also need to unbalance the signals coming from the professional source equipment. Unfortunately, apart from the use of transformers, no one passive method of unbalancing a signal is appropriate for all possible sources. Some hookup methods that are perfectly OK with some sources will damage others or distort their signals; other hookup methods that are perfectly OK with some other sources can cause no signal at all (except for a tiny bit of noisy leakage) to appear in your recorder with other sources.
In professional situations there are two ways that this problem is typically handled. One is with an active interface (often these are built so that the same unit converts two channels of professional/balanced signals to consumer/unbalanced and vice versa), but that typically requires AC powering. The other approach is with a transformer box (a nice example being
http://www.jensen-transformers.com/pc2xr.html), which requires no powering and is the cleanest, most general and reliable solution, although the leads from the transformers to the inputs of your consumer device must be kept quite short.
--best regards